They _______ their way into the house, if I had not called for help.

The correct answer is: b) would have been forced

In this sentence, the use of "if I had not called for help" suggests a hypothetical or unreal condition in the past. When constructing hypothetical situations in the past, we use the past perfect tense (had + past participle) in the conditional clause. In this case, the conditional clause is "if I had not called for help."

The main clause of the sentence describes the result or consequence of the hypothetical situation. In this case, it suggests that the action of being forced into the house didn't actually happen, but it's a hypothetical scenario. Therefore, the correct form to use in the main clause is "would have been forced" (the past participle form of "force" in the past perfect tense), making option b) the correct choice.

So, the complete sentence is: "They would have been forced their way into the house if I had not called for help."