If they _________ that the weather was going to be so bad, they wouldn't have gone to the beach.

The correct answer is 'C: Had known.' In this sentence, the structure is a third conditional, which is used for unreal situations in the past. 'Had known' is the correct past perfect form that matches the conditional structure. It indicates that if they had possessed the knowledge about the bad weather in advance, they wouldn't have gone to the beach. Options A, B, and D do not correctly fit the past perfect tense required in this context. 'Know' (Option A) is present tense, 'Knew' (Option B) is simple past tense, and 'Were knowing' (Option D) is incorrect and does not convey the intended meaning of the sentence in the context of a third conditional.