He proved utterly capricious in his dealings with his friends.

Answer: Option B

Capricious : given to sudden and unaccountable changes of mood or behaviour.
Helpful : giving or ready to give help.
Steadfast : resolutely or dutifully firm and unwavering.
Understanding : the ability to understand something; comprehension.
Obstinate : stubbornly refusing to change one's opinion or chosen course of action, despite attempts to persuade one to do so.

Antonym of Capricious is Steadfast