It is important that John _____________ his resume immediately.

The correct choice is "send" because it follows the structure of the subjunctive mood, which is used after certain expressions that convey necessity or urgency. In this case, the phrase "It is important that John" indicates a recommendation or necessity, prompting the verb that follows to appear in its base form, which is "send." The subjunctive mood in English often uses the base form of the verb regardless of the subject, which is why "send" is appropriate here, while the other options do not fit the grammatical structure required after "that."