The answer is B
As L is undecidable , take any case like Recursive Enumerable languages
RE can be infinite , so statement I is true
Recursive languges are the proper subset of RE, so statement II is true
If Complement of RE is also Undecidable then RE becomes Recursive that makes RE as Decidable languages which is not true. So statement III is false
So statement I and II are true but III is false
Others Opinions are welcome
-angelina