The correct answer is C. servile.
Servile means “subservient or excessively submissive.” In this context, the speaker is saying that they don’t understand why the person should be so submissive to their brother just because he is rich. This could be because the speaker believes that everyone should be treated equally, regardless of their wealth, or because they think that the person is being too eager to please their brother in order to get something from him.
The other options are incorrect because they do not accurately describe the person’s behavior. Kind means “having or showing a friendly, generous, and considerate nature.” Unfriendly means “not friendly or sociable.” Offensive means “causing someone to feel deeply hurt, upset, or angry.”