This question revolves around conditional language. That is a sign that it might be a good idea to diagram. So let's try it out. Also, please note that on test day and in practice, this should be abbreviated. Just for clarity, I will write out terms in full.
City builds proposed convention center - > Several conventions
Several large conventions - > Total number of visitors increase
Total number of visitors increase - > Tax revenues increase
Then the argument concludes: City builds proposed convention center - > tax revenues increase
This may all seem reasonable and appear to be a continuous flow of conditional statements, however, the chain broke between the first and second conditional statement. The sufficient condition of the first statement is several conventions, while the necessary condition of the second statement is several "large" conventions.
We cannot jump from several conventions to several "large" conventions without linking these somehow. This is what answer choice E does by saying the following:
Several conventions - > Large conventions
This is the missing link.
However, answer choice B provides:
Total number of visitors increases - > Amount spent by visitors increases
This cannot be linked anywhere in our conditional chain to guarantee the conclusion. An increase in amount spent by visitors does not guarantee an increase in city's tax revenue according to the conditions provided.
I hope this helps. Please let me know if you have any other questions.