The closest synonym for the word adherent is: A. Proponent.
Adherent basically is similar to the loyalty and attachment to a party
Proponent means a person who advocates a theory proposal - defender, follower, enthusiast
Informant is someone who tells someone something
Agnostic is someone who believes nothing is known
Financier is someone concerned with managing money
Therefore we can see proponent would be the closest synonym as a proponent advocates a theory proposal so they stick to it, are loyal, supportive and backs up something.
This is similar to adherent which means someone who supports a particular party.
