B. (her)
it can't be A because mom is not a 'he' so wouldn't be 'his'
it can't be C because 'his and her' only accounts for 1 'he', but there are 2 brothers.
it wouldn't be D either because 'loves' is used only for a singular subject. I can't say "they loves chocolate." Remember, only singular subjects, like "she loves chocolates" (singular 'she')