The sentence which uses the subjunctive mood correctly is "It's imperative that everyone memorize the time tables," as stated in option C and explained below.
In English the subjunctive mood is used to express the following:
The subjunctive mood accompanies verbs or phrases such as:
When accompanying phrases such as "it's imperative," the verb in the subjunctive mood must remain in its base form. That is why option C is correct.
Learn more about the subjunctive mood here:
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