Which compound sentence is not correctly punctuated?

AHis new watch is self-winding; it still needs a battery.
BHis new watch is self-winding, but it still needs a battery.
CHis new watch is self-winding but it still needs a battery.

Verified Answer :

b. His new watch is self-winding, but it still needs a battery.
It contains a conjunction between the two clauses and has a comma too. 
Hope this helps. 

The correct answer is b because the watch is self winding, however, the watch will not self-wind unless batteries are put in. Hope I helped! Good luck!