Final answer:
The correct choice is 'arrive' because the sentence is in the subjunctive mood, which is indicated by the expression 'It’s best that' and requires the base form of the verb.
Step-by-step explanation:
The correct verb to complete the sentence "It’s best that John ___ by 3:00 so that we can leave before traffic gets heavy" is arrive. This is because the sentence requires the subjunctive mood, which is used to express wishes, suggestions, or requirements. In this case, the subjunctive mood is triggered by the expression 'It’s best that,' which is often followed by a verb in the base form (which for the verb 'to arrive' is 'arrive'), to indicate a suggestion or requirement. Therefore, the correct option is B: arrive.