@AmyDoval I've run into the same problem, and for me, it has helped to try and differentiate between the language "if and only if" and just "only if." Annoyingly, "only if" is the same part as "then" in a one way conditional. So, "if A, then B" and "A only if B" are equivalent statements.

A good example might be:

if [one is in Texas] then [one is in the United States]

which would be functionally equivalent to:

[one is in Texas] only if [one is in the United States]

the above examples would not be equivalent to the statement

[one is in Texas] if and only if [one is in the United States]

Because biconditional statements, (<->) are always fully reversible while maintaining their meanings, to reverse this would be to say:

[one is in the United States] if and only if [one is in Texas]

which simply isn't true.

In applying this logic to question 2, we can see that the premise we are targeting says something along the lines of:

major changes --> complex reasoning

so when we take the contrapositive, we need an assumption that fits the structure:

complex reasoning --> major changes

Answer A (because it uses the language "only if") is still structured to say "major changes --> complex reasoning" but this functionally says "all animals that can make major changes are capable of complex reasoning" leaving room for other animals that are not capable of making major changes to still be capable of complex reasoning. The statement can only be negated if an animal is capable of making changes but NOT capable of complex thought, and because the author states the premise that reptiles are not capable of making changes, the statement ends up being irrelevant.