Meaning: 'Aims' means to have a specific goal or purpose, indicating that the researchers are focused on assessing the suitability of watermeal.

Wrong options:

• enters: The word 'enters' implies beginning or coming into a space, which does not convey the intention of the research.
• fits: The word 'fits' suggests compatibility, but it does not express the goal of the research clearly.
• works: The word 'works' implies functioning or operating, which is too vague and does not specify the research's objective.

Correct answer: source

Explanation: The term 'source' is appropriate here as it refers to the origin or provider of something, which aligns with the context of food and oxygen production for astronauts.

Meaning: 'Source' means a place or thing from which something originates, indicating that watermeal could provide food and oxygen.

Wrong options:

• root: The word 'root' refers to the part of a plant that attaches it to the ground, which does not fit the context of food production.
• process: The word 'process' refers to a series of actions or steps taken to achieve a particular end, which does not directly relate to being a provider of food.
• cause: The word 'cause' implies a reason for something happening, which does not fit the context of providing food.

Correct answer: ideal

Explanation: The word 'ideal' fits perfectly as it describes something that is most suitable or perfect for a particular purpose, which is the case for watermeal in space cultivation.

Meaning: 'Ideal' means the most suitable or perfect option, indicating that watermeal is an excellent candidate for cultivation in space.

Wrong options:

• articulate: The word 'articulate' refers to the ability to express ideas clearly, which does not apply to describing a candidate for cultivation.
• exhaustive: The word 'exhaustive' means thorough and complete, which does not convey suitability in this context.
• absolute: The word 'absolute' means complete or total, which does not accurately describe the suitability of watermeal.

Correct answer: successful

Explanation: The word 'successful' is the correct choice as it describes the condition of achieving a desired outcome, fitting grammatically and contextually in the sentence.

Meaning: 'Successful' means having achieved a desired aim or result, indicating that if the research achieves its goals, watermeal could be vital for life-support systems.

Wrong options:

• successful: The phrase 'is successful' is grammatically incorrect in this context as it adds unnecessary auxiliary verb 'is'.
• is succeeded: The phrase 'is succeeded' implies being followed by something else, which does not fit the context of achieving success.
• succeeds: The word 'succeeds' is in the present tense and does not fit the conditional structure of the sentence.

Correct answer: under

Explanation: The word 'under' is appropriate as it indicates the conditions in which the research is conducted, fitting grammatically and contextually with the phrase 'extreme conditions'.

Meaning: 'Under' means in or into a position below or beneath something, indicating the conditions being studied in the research.

Wrong options:

• beyond: The word 'beyond' implies a position further away or outside of something, which does not fit the context of studying conditions.
• within: The word 'within' suggests being inside something, which does not accurately describe the external conditions being researched.
• throughout: The word 'throughout' indicates something happening in every part of a space or time, which does not align with the specific focus on extreme conditions.