A.I. Explanation:

Correct answer: order

Explanation: The word 'order' fits grammatically as it indicates a sequence or arrangement necessary for solving problems, aligning with the context of fluid intelligence and logical thinking.

Meaning: In this context, 'order' refers to the arrangement or sequence in which thoughts or actions are organized to effectively tackle new challenges.

Wrong options:

• relation: The term 'relation' suggests a connection but does not imply the necessary sequence for problem-solving, making it contextually inappropriate.
• case: The word 'case' refers to a specific instance or situation, which does not align with the concept of logical arrangement needed here.
• part: The term 'part' implies a portion of something, which does not convey the idea of sequence or arrangement required for solving problems.

Correct answer: peaks

Explanation: The verb 'peaks' is appropriate as it indicates reaching a maximum point, which aligns with the context of fluid intelligence in young adulthood, fitting grammatically and contextually.

Meaning: 'Peaks' means to reach the highest point or level, indicating that fluid intelligence is at its best during young adulthood.

Wrong options:

• plunges: The word 'plunges' suggests a sudden drop, which contradicts the idea of reaching a maximum level of intelligence.
• halts: The term 'halts' implies stopping, which does not fit the context of intelligence reaching a peak.
• stagnates: 'Stagnates' means to stop developing or progressing, which is not suitable as it does not convey the idea of reaching a high point.

Correct answer: decline

Explanation: The word 'decline' fits grammatically and contextually as it indicates a gradual decrease in fluid intelligence as we age, aligning with the overall message of the passage.

Meaning: 'Decline' refers to a gradual decrease or reduction, indicating that fluid intelligence diminishes over time as we grow older.

Wrong options:

• influx: The term 'influx' refers to an arrival or increase, which contradicts the idea of a decrease in intelligence.
• peak: The word 'peak' suggests reaching a maximum, which is the opposite of what is being described in terms of aging and intelligence.
• increase: 'Increase' implies growth or enhancement, which does not align with the context of fluid intelligence diminishing with age.

Correct answer: while

Explanation: The word 'while' is suitable as it introduces a contrast between the inevitability of aging and the potential for changes in brain function, fitting grammatically and contextually.

Meaning: 'While' indicates a simultaneous occurrence or contrast, suggesting that although aging is unavoidable, brain function changes may not be.

Wrong options:

• if: The term 'if' introduces a conditional statement, which does not fit the context of contrasting inevitability with potential findings.
• when: The word 'when' suggests a specific time, which does not convey the contrast needed in this context.
• whether: 'Whether' implies a choice or uncertainty, which does not align with the definitive nature of the statements regarding aging and brain function.