Tourism is the industry in the world.
a. third-largest
b. second-largest
c. largest
d. fourth-largest

This is really hard to estimate, as it's not exactly perfectly clear which parts of economy are tourism (which of the transportations? are museums tourism or education?)  and we have only estimates. Some of the estimates I have seen judge tourism to be only 6th ot 7th industry (but none of the options propose this!), but others say that it's third, so I'd go with a) - but definitely not c) or b). First is still food (agriculture, farming, etc)