Contrapositive: an increase in visitors is necessary (as well as sufficient) to increase revenues. But adding a necessary condition to increase revenues can’t possibly help explain how some other condition (building the center) is sufficient to increase revenues.

The premises already say that if the number of visitors increases, tax revenues will certainly increase. (B) might explain the mechanism for that revenue increase, but it doesn’t affect the logic of the argument. We still don’t know if building the center will increase visitors.

This says building the convention center is necessary to increase revenues, but we need to reach the conclusion that building the center is sufficient to increase revenues. (C) leaves open the possibility that building the center won’t be enough on its own to increase revenues.

What any specific visitors do is irrelevant. All that matters is whether the total number of visitors increases. And as as long as large conventions are held, the total number will increase. The problem is that we don’t know if building the center will lead to large conventions.

This completes the conditional chain in the premises. Building the convention center means these organizations will hold their conventions there. Assuming those conventions are large, revenues will increase. So building the convention center means revenues will increase.