I don't see why he should be so obsequious to his brother just because the latter is rich.

Answer: Option C

Kind : a group of people or things having similar characteristics.
Unfriendly : not friendly.
Servile : having or showing an excessive willingness to serve or please others.
Offensive : causing someone to feel resentful, upset, or annoyed.
Obsequious : obedient or attentive to an excessive or servile degree.

Synonym of Obsequious is Servile