How will the right answer fit in terms of support and conclusion?

Only the right answer will add support for the conclusion.

Highlight the main conclusion in the passage, if there is one:

siblings of first-born children will tend to be more adventurous than will be the first-borns.

Psychologist: [BACKGROUND]. [SUPPORT]. [SUPPORT]. Hence, our study suggests that [CONCLUSION].

Map the wording of the answers to the wording of the passage:

(A) Some of the great creative geniuses…

Stop. What do “creative geniuses” have to do with being “adventurous”? That’s what the conclusion is about, so I’d say this answer is changing the subject big time.

(B) …the greater one’s cognitive plasticity.

This sticks to wording used in the passage, but it doesn’t connect to the conclusion well at all. What does this tell us about the “siblings” themselves, or about being “adventurous”? That’s what the conclusion we want to support is about.

(C) …a correlation between cognitive plasticity and the willingness to take risks.

Now we’re talking. The author only said “it is reasonable to think” that siblings would be “more adventurous”. They didn’t actually give evidence for that. The study only shows siblings have greater “cognitive plasticity”. If other studies have made the connection, then the author’s conclusion is definitely more reasonable.

(D) …several industry leaders have older siblings.

I think we’ll agree that “industry leaders” is coming out of nowhere. You’d need your own assumption to connect that to “adventurous”, so this has to be eliminated.

(E) …had characterized themselves…

The conclusion is only about how people actually are, not how they “characterized themselves”. And “adaptable” has a broader meaning than just “open to new ideas”, so that wording doesn’t map too tightly either.

(C) is the correct answer.