The correct option is
C I agree with your idea and I find it interesting.
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: This is a simple sentence as it contain a single clause viz, "I agree with it" . It has a single subject ("I") and predicate ("agree with it"). The words, "Despite finding your idea interesting" is a phrase. It is not a clause as it does not contain a subject and a predicate. Hence, this option is incorrect.
Option B: "If" is a subordinating conjunction which expresses a condition. The clause, "I find it interesting" is an independent clause. The clause, "If I agree with your idea" 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 C: The two clauses viz (i) "I agree with your idea" and (ii) " I find it interesting", can stand independently and make good sense. They are independent clausesconnected using the co-ordinating conjunction-"and". Hence, this is a compound sentence and this answer is correct.
Option D: The word, "because" is a subordinating conjunction used to indicate cause/ reason. The clause, "I agree with your idea" is an independent clause. The clause, "because I find it interesting" is a dependent clause. Hence, this is a complex sentence and this option is incorrect.