Meaning: 'Organisms' are living beings, such as plants, animals, and microorganisms.

Wrong options:

• prospect: The word 'prospect' refers to the possibility or likelihood of something happening, which does not relate to the context of life forms.
• departments: 'Departments' refers to divisions within an organization or field, which is unrelated to the biological context of life.
• supplements: 'Supplements' are additions that enhance or complete something, but they do not refer to living entities and thus do not fit here.

Correct answer: majority

Explanation: The word 'majority' indicates the greater part of something, which is appropriate here as it describes the predominant elements that constitute life's molecular structure. It fits grammatically as a noun that aligns with the context of discussing the elements.

Meaning: 'Majority' refers to the greater number or part of something.

Wrong options:

• prospect: 'Prospect' does not fit as it refers to a possibility rather than a quantity of elements.
• departments: 'Departments' is unrelated to the context of elements and does not convey a sense of quantity.
• supplements: 'Supplements' does not fit as it implies additions rather than a primary quantity.

Correct answer: regardless

Explanation: The term 'regardless' serves as an adverb indicating that something is true without consideration of other factors. In this context, it emphasizes that the search for extraterrestrial life will continue irrespective of its chemical basis, fitting both grammatically and contextually.

Meaning: 'Regardless' means without paying attention to the present situation; despite the circumstances.

Wrong options:

• prospect: 'Prospect' does not fit as it refers to a possibility rather than a condition of the search.
• departments: 'Departments' is unrelated to the context and does not convey the intended meaning of ignoring conditions.
• supplements: 'Supplements' does not fit as it implies additions rather than a disregard for conditions.

Correct answer: reaches

Explanation: The word 'reaches' refers to distant areas or extents, which is appropriate in the context of discussing the far parts of our galaxy. It fits grammatically as a plural noun that aligns with the idea of exploring vast distances.

Meaning: 'Reaches' refers to the distant areas or extents of something.

Wrong options:

• prospect: 'Prospect' does not fit as it refers to a possibility rather than a physical area.
• departments: 'Departments' is unrelated to the context of distance and does not convey the intended meaning of areas.
• supplements: 'Supplements' does not fit as it implies additions rather than areas or extents.