Which one of the following is most stable?

Option A is a tertiary ($$3^\circ$$) allylic carbocation. The positive charge sits on a carbon attached to three other carbons, and it is directly adjacent to a conjugated double-bond system. Furthermore, the double bond is in conjugation with a phenyl ($$\text{Ph}$$) ring, which allows extensive delocalization of the positive charge throughout the aromatic ring via resonance. Hence, it is the most stable.