Meaning: 'Requires' means to need something for a particular purpose, indicating that life needs these six ingredients to exist.

Wrong options:

• outsiders: 'Outsiders' refers to people not belonging to a particular group, which does not fit the context of ingredients necessary for life.
• imbibers: 'Imbibers' refers to those who drink, which is irrelevant in this context as it does not relate to the ingredients for life.
• buyers: 'Buyers' refers to people who purchase goods, which is unrelated to the concept of life needing certain ingredients.

Correct answer: phosphorus

Explanation: The noun 'phosphorus' fits grammatically as it is a singular noun that completes the list of essential ingredients, maintaining the parallel structure of the sentence.

Meaning: 'Phosphorus' is a chemical element essential for life, particularly in DNA and RNA structures.

Wrong options:

• preponderates: 'Preponderates' means to be greater in number or importance, which does not fit as it is not a substance or ingredient.
• artlessness: 'Artlessness' refers to simplicity or innocence, which is irrelevant in the context of chemical ingredients for life.
• tuberculous: 'Tuberculous' relates to a disease caused by tuberculosis, which does not fit the context of essential life ingredients.

Correct answer: elements

Explanation: The noun 'elements' fits grammatically as it is plural and refers to the six basic ingredients mentioned, aligning with the context of fundamental components of life.

Meaning: 'Elements' refers to the basic substances that make up matter, in this case, the essential ingredients for life.

Wrong options:

• spectacles: 'Spectacles' refers to glasses for vision, which is unrelated to the context of life ingredients.
• shepherdess: 'Shepherdess' refers to a female shepherd, which does not relate to the scientific context of elements necessary for life.
• coniferous: 'Coniferous' describes trees that bear cones, which is not relevant to the discussion of life’s basic ingredients.

Correct answer: building

Explanation: The adjective 'building' fits grammatically as it describes the blocks that are fundamental to constructing DNA and RNA, aligning contextually with the idea of foundational components.

Meaning: 'Building' in this context means foundational or essential, indicating that these elements are crucial for constructing biological molecules.

Wrong options:

• perplexing: 'Perplexing' means confusing, which does not fit the context of describing essential components for life.
• shrinking: 'Shrinking' refers to becoming smaller, which is irrelevant in the context of building blocks for DNA and RNA.
• whipping: 'Whipping' refers to a rapid movement or action, which does not relate to the concept of essential components for life.

Correct answer: researchers

Explanation: The noun 'researchers' fits grammatically as it is plural and refers to the scientists conducting the study, aligning contextually with the narrative about scientific discovery.

Meaning: 'Researchers' are individuals who conduct scientific studies to discover new information, in this case, about bacteria and arsenic.

Wrong options:

• strap: 'Strap' refers to a strip of material, which does not fit the context of individuals conducting research.
• kerb: 'Kerb' refers to the edge of a pavement, which is unrelated to the context of scientific research.
• nub: 'Nub' refers to a small lump or piece, which does not relate to the context of people involved in research.

Correct answer: confirmed

Explanation: The verb 'confirmed' fits grammatically as it is in the past tense, matching the context of the tests that validated the findings about the bacteria, and it logically follows the narrative of scientific validation.

Meaning: 'Confirmed' means to establish the truth or correctness of something, indicating that the tests validated the bacteria's ability to use arsenic instead of phosphorus.

Wrong options:

• harangued: 'Harangued' means to lecture someone aggressively, which does not fit the context of scientific testing.
• confidant: 'Confidant' refers to a person with whom one shares secrets, which is irrelevant in the context of scientific results.
• gummed: 'Gummed' refers to sticking or adhering, which does not relate to the context of confirming scientific findings.