[Taqwus]

For most such items, it's B. Usually you don't even need any magic skills, let alone skills sufficient to cast the spell -- although the description for the Broken Mirror tells me that it's possible for an item to have such a path requirement to work. Fatigue is also dropped to 5, IIRC, for item use, instead of the full fatigue for a non-assisted spell cast.

Note that because it's treated as a spell cast, it takes a full action -- ex. you cannot give four Rods of the Phoenix and Boots of Quickness to a Nataraja, and expect him to cast Incinerate 8x/turn.

Some items auto-cast at the beginning of battle -- ex. a Crystal Shield auto-casts Power of the Spheres, a Staff of Storms auto-casts Storm, et al. Others auto-cast each turn, ex. a Spirit Helm tosses lightning bolts without requiring the user's action.