C is correct.
Premises:
- a play must give pleasure to its immediate audience, unlike novels who have months/years to please readers.
- a play must give pleasure to its immediate audience by reflecting the concerns and values of that audience
Conclusion:
- Successful drama of the Restoration period in(is??) a good index to the typical tastes and attitudes of its time.
Assumption : Successful drama of the Restoration period agreed with tastes and attitudes of many people. i.e. Many people of the Restoration period enjoyed the drama.
A) plays written for restoration audiences do not appeal to modern audiences. >> Argument doesn't say this.
B) plays are superior to novels as a form of narrative art >> dramas and plays are compared, but author doesn't say which one is superior
C) Restoration audiences were representative of the whole population of their time >> Correct. This is the assumption. Negating this (very few people enjoyed Restoration drama) we can shatter the conclusion.
D) playgoers and novel readers are typically distinct and exclusive groups. >> Out of scope
E) Restoration drama achieved popular success at the expense of critical success >> Argument doesn't say this.