In my understanding:

would fail ... is : is incorrect

would must be together with were (present unreal condition sentence)

So the correct use here is : will .... is. Eliminate A,E

Should is used incorrectly here. Should can replace "if" in the condition sentence :

...should the density of mosquitoes in that region be more numerous than 100,000 per square mile.
Eliminate D.

B use the incorrect pair : will ....were

Thus C is the best choice.


About greater than/ more numerous than/more than, IMO:

Density of A is greater than density of B.
We care about degree, the size here.
Take this example:
No. of A is greater than no. of B.
No. of A : 1 object ; no. of B: one object;
1 object cannot be more numerous than 1 object, but 1 object can be greater than 1 object.
density of mosquito here is an object.
To make it clearer:
2 is greater than 1
2: 1 number ---- 1: 1 number
1 =1, so 2 cannot be more numerous than 1 ( the quantity is equal here), but 2 is greater than 1 ( compare in value)

If A is a countable noun, then we can use "numerous":
In my room, mosquitos are more numerous than fleas.
We care about quantity here.

I think more than is also about quantity. But it can be use for both countable and uncountable noun.
I have more water/house/car/children/money than you.
"More than" has nothing to do with degree or size.

Correct me if I'm wrong.

Thank you for your efforts. I will follow this but only an expert can validate this.

To help you further why more than can be used for countable and uncountable..

normal comparative superlative

Many MORE most
much MORE most

MORE is comparative of many and MUCH both.

GMATMadeEasy , you might want to remove the typo 'correct' at the end of the option C in the first post. This is for the benefit of others who want to try the question on their own.