Someone is said to have a vested interest if
a. what happens will not have any impact on their lives.
b. their family or loved ones will not be affected by an event or situation.
c. they understand an event or situation completely.
d. they resist any change that threatens their security or standard of living.

The answer is the option d, they resist any change that threatens their security or standard of living. In other words, when a person has a vested interest, then that person will select the option that benefits him or her the most. For example, a teenager that cleans his room and helps at home because he expects to get something from his parents has a vested interest. He is not cleaning the room because he likes the an organized space, he does it because he wants to get an advantage from it.

answer; researchers typically cannot control people's sexual behaviors as would be necessary in an experiment;

idk