The correct option is
A He did not expect anything in return for his help.
A simple sentence has only one subject and predicate. It has a finite verb with a completed thought. A compound sentence has at least two parts (also referred to as a clause). Each part has its own subject and predicate and makes good sense by itself. Hence each clause is independent of the other and called the main clause or independent clause. The main clauses are connected using a co-ordinating conjunction (for, and, nor, but, or, yet, so).A complex sentence has at least one main clause (which is an independent clause) and one or more dependent clauses, connected using a subordinate conjunction. The dependent clause cannot stand by itself and make good sense. It depends upon the main clause.
Option A: The word, "though" is a subordinating conjunction which connects dependent and independent clauses. The clause, "He did not expect anything in return" is an independent clause. The clause, "Though he helped me" cannot stand by itself and make good sense. It is a dependent clause. Hence, this is a complex sentence and this option is incorrect.
Option B: The two clauses viz (i) "He did not expect anything in return" and (ii) "He helped me", can stand independently and make good sense. They are independent clausesconnected using the co-ordinating conjunction-"but". Hence, this is a compound sentence and this answer is incorrect.
Option C: This sentence has only one clause i.e. "He did not expect anything". It has a single subject-"He", and predicate-"did not expect anything". The words, "in return for his help" is a phrase and not a clause. Hence, this is a simple sentence and this option is correct.
Option D is incorrect as option C is the correct answer.