Write through protocol updates (or invalidates) the shared data, as and when needed. It asks to update/invalidate via broadcasting (hence, not efficient, as it creates traffic)
Buffered write is an added functionality to write through cache. (Hence, not efficient, again)
Write back protocol is based on ownership of a block. (Read page 454 of Hamacher, 6th Ed). It also uses broadcast and creates traffic, hence, not efficient.
Though, the traffic generated is less than Write through.
Snooping:
the abilities of cache controllers to observe the activity on the bus and take appropriate actions are called snoopy-cache techniques.
For performance reasons, it is important that the snooping function not interfere with the normal operation of a processor and its cache.
Hence, efficient.
Option A