Combine the two simple sentences below to form a simple sentence:
They must practice hard. They will win.

The correct option is D They must practice hard to win.
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, "If" is a subordinating conjunction which connects dependent and independent clauses. The clause, "they will win" is an independent clause. The clause, "If they practice hard" 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) "They must practice hard" and (ii) "they will win", can stand independently and make good sense. They are independent clauses connected using the co-ordinating conjunction-"and". Hence, this is a compound sentence and this answer is incorrect.

Option C: The word, "unless" is a subordinating conjunction which connects dependent and independent clauses. The clause, "they cannot win" is an independent clause. The clause, "Unless they practice hard" 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 D: This sentence has only one clause with a single subject ("They") and predicate ("must practice hard"). The words, "to win" is an infinitive, which functions as an adverb. It is not a clause. Hence, this is a simple sentence and this option is correct.