Answer is C :
The reason for it since they said classical Reader Writer problem it mean First Reader Writer problem .
The first Reader Writer problem says that : At any time, on a shared resources multiple reader can read it. In between if any writer comes then it wont be allowed unless last reader complete its reading. (so writer starves here )
So following the above thing :
S1= Wait(wrt) implies that as soon as the first reader come, it should lock resources so that no writer can access it.
S2 = signal (mutex ) implies that mutex is made to 1, so other reader can entry into entry section. Multiple reading are allowed. (only 1 at a time can be in entry section )
S3 = wait (mutex) implies that at a time only one process can be in Exit section and semaphore mutex is used to implement it (only 1 at a time can be in exit section)
S4 = signal (wrt ) implies that when last reader has completed reading, it should unlock resources so that any writer waiting for it can use it .
The above values of S1, S2, S3, and S4 are all nothing but implementing first reader writer problem :)