If the nurse _________________ the patient was going to throw up. She would have brought a basin.

The correct choice, "had known," fits the sentence structure appropriately because it follows the rule for forming a third conditional statement. This type of conditional is used to discuss hypothetical situations in the past that did not happen.

In this case, the nurse did not know that the patient was going to throw up, which is reflected in the phrase "If the nurse had known." The past perfect tense "had known" implies that there was a lack of knowledge in the past, which would have resulted in the nurse taking a specific action—bringing a basin. This structure emphasizes the conditional nature of the statement; since the past condition (the nurse knowing) was not met, the nurse did not take the action (bringing the basin).

This grammatical construction highlights both the connection between cause and effect in past scenarios and the use of proper tense forms to convey such hypothetical situations effectively.