plz help me.I came across these sentence and don't know how to understand them

1)what would you do if your car was stolen( why was is correct in this case?)

2) Let's listen to____ the piano.

a)what he plays c) him playing

b)his playing d) Mary's playing

thanks in advance
1) "was" is simply a less formal form in this structure. Many speakers use this less formal form in everyday conversations.
2) c) him playing. Think of it as "Let's listen to him while he is playing the piano".

thankyou very much,CalifJim