Which word of the ones is incorrect in the following sentence? The formula used in the study calls for either peroxide or metaldehyde, but metaldehyde was not always available.

The wrong word in the whole sentence it was. The tense that is used in the sentence is in past form based on the verbs that are used. The reason why was is the problematic word is that of what the sentence is trying to connote in general.

It says that peroxide or metaldehyde may be used but there are times when metaldehyde is not always available. The reason why is, is the better term to use is the word always. Was it not the term that is often used together with the word always. The sentence will make sense more if the verb “is” is used.