Melee game.

Player A controls King Roberts Host.

Player B has Crown Regent title.

Player C initiates Power challenge against Player B.

Player B use his title to make player C attack another opponent.

Since Player A controls King Roberts Host which say While King Roberts Host is standing, opponents cannot initiate Power challenge against you.

Can Player C attack player B.

Thanks in advance.

From the Core Set rules, p. 18:

"After a player initiates a challenge and declares a target and attackers, you may use this title to make that player choose a new target for the attack! That player must choose a legal target. If there are no other legal targets, the attack goes through against the original target. "

So in your scenario, since C can't attack A (or himself), the attack goes through against B and B wasted his use of the Title.