Question

Tourism is the _______ industry in the world.

Answer 1

Well, tourism is the largest commercial service industry in the world.

Some sources list tourism at the largest indistry in the world, but this is difficult to measure. Generally, most sources say that food is the largest industry, followed by either Tourism or oil industry or communication.

The problem with this is that it's hard to distinguish: communication and oil are also used in tourism so the same activity can be a part of more than one sector. I would  say that D is the best answer, but the question should specify the source it is referring to.