Answer:
Singular = strange
approbation = praise
mercenary = flattering
Step-by-step explanation:
The meaning of singular as an adjective is that it is unique and strange which is the similar word in this list of options, being strange is not to be common or like the rest.
The meaning of approbation is to categorize something as good or acceptable, that is quite similar to what we do when we praise someone and say a good and positive thing about something.
The meaning of mercenary is a selfish person who doesn't worry about anything else but their own, flattering in the other hand is to make something look greater and more special which could be related to the selfishness of the mercenary.