b. the marginal benefit of the trip is greater than $200.

Reason: The marginal benefit is the additional benefit that the person receives when he consumes one extra unit of a good. The marginal cost on the other hand is the additional cost incurred in buying or producing one extra unit of a good. The marginal benefit from going to the beach must have exceeded the marginal cost of $200 for Joe, as he decides to go for a beach trip in the weekend. If the marginal benefit would have been less than $200, then Joe would have decided otherwise.