Which word is used correctly in the following sentence?

The choice of "were" in the sentence is appropriate when discussing actions, states, or conditions that occurred in the past with a plural subject. This word functions as the past tense of "are" and is used correctly in contexts where multiple subjects are being referenced or when speaking about previous agreements or situations.

Using "were" correctly conveys the intended meaning, especially in a context emphasizing that more than one person or thing was involved in an action or state at that time. For example, in a sentence like "The students were studying diligently," "were" accurately identifies the past activity of multiple students.

The other words—thoroughly, student, and procedure—can also be contextually appropriate in different sentences, but they do not fulfill the specific grammatical requirement in the context typically associated with a correct choice for this type of question.