The verb in the sentence "The chef put a chili pepper in the sauce to add flavor" is in the A. Active Voice
It is in the active voice because the subject is the one doing the action.
chef - subject of the sentence
put - verb
Active voice because he, the chef, is the one who actually does the act of putting the chili pepper.
Additional note: Passive voice is used when the subject is being acted-upon or is not directly involve in the action.
Example of passive voice using the above sentence.
The chili pepper is being put in the sauce.
chili pepper - subject
is being put - verb
Clearly, the subject is not actively participating in the performance of the verb.