He must run fast or he will not catch the train.
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 on the main clause.
Option A: This sentence has only one clause with a single subject ("He") and predicate ("must hurry"). The words, "to catch the train" is a phrase and not a clause. Hence, this is a simple sentence and this option is incorrect.
Option B: The two clauses viz (i) "He must run fast" and (ii) "He will not catch his train", can stand independently and make good sense. They are independent clausesconnected using the co-ordinating conjunction-"or". Hence, this is a compound sentence and this answer is correct.
In options C and D, "If" is a subordinating conjunction which connects dependent and independent clauses.
Option C: The clause, "He must run fast" is an independent clause. The clause, "if he wants to catch the train" cannot stand by itself and make good sense. It is a dependent clause.
Option D: The clause, "He will not catch the train" is an independent clause. The clause, "if he does not hurry" cannot stand by itself and make good sense. It is a dependent clause.
Hence, both options C and D are complex sentences and these options are incorrect.