What is the correct verb tense for the sentence regarding experiment design?

The correct choice employs the subjunctive mood, which is used to express hypothetical situations or conditions that are contrary to reality. In this case, "If the experiment were not designed this way" illustrates that the statement is discussing a hypothetical scenario regarding the design of the experiment, suggesting an alternative that is not currently true.

This subjunctive tense is appropriate when introducing conditions that are not real or that are contrary to the existing state of affairs. It conveys an assumption about a situation that is imagined rather than factual. The use of "were" instead of "was" is a key indicator of this mood and indicates that the sentence is not referring to a past event but rather an imagined or theoretical condition.

In contrast, the other choices use different tenses or structures that do not appropriately capture the intended meaning of discussing an unreal condition. For example, they either imply a real situation by using an indicative mood or do not fit the context of hypothesizing a change in the experimental design. This distinction emphasizes the importance of using the correct verb tense to convey the intended meaning clearly in academic writing.