Answer:
5. Only a credulous person could believe her tale.
6. He affected a complete change in the appearance of the compound by cooling it.
7. All except her have chosen their research topics for next year.
Step-by-step explanation:
The adjective "credible" is the same as "believable". On the other hand, "credulous" refers to someone who believes things easily. Thus, for sentence 5, "credulous" is the best option. Only someone who believes things easily would believe her tale.
"Effect" is a noun, and it is the same as "consequence" or "result". What we need for sentence 6 is a verb. "Affect" is a verb that means to have an impact on or to change something. Thus, "affect" is the best option. He changed the appearance of the compound - he affected it.
Finally, after "except", it is best to use the object pronoun "her" instead of the subject pronoun "she".