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

The correct choice reflects a conditional statement that indicates a hypothetical situation in the past. The phrase "If they _________ that the weather was going to be so bad, they wouldn't have gone to the beach" implies that the speaker is discussing a scenario that did not happen, specifically concerning knowledge of the weather conditions prior to their decision.

In this case, "had known" is the past perfect form, which is used to indicate an event that happened before another past event. Here, the understanding is that if they had known about the bad weather in advance, their decision to go to the beach would have been different. This construction conveys a sense of regret or reflection on a past choice based on information that was not available at that time.

The other options do not fit this context. "Know" is present tense and would imply that the situation is current, which does not align with the conditional statement about a past event. "Knew" indicates a simple past but lacks the necessary perfect aspect to correctly convey the condition leading up to another past action. "Were knowing" is grammatically incorrect and not commonly used in English as a continuous form in this context. Thus, "had known" is the appropriate choice for creating a coherent and grammatically correct