[rewboss]

None of them -- the question is worded incorrectly, but it's a common error.

The answer they are looking for is probably C. The sentence is actually two sentences combined, like this:

Disease can be as harmful to [...] a nation as wars or economic depression
if disease is not more harmful to [...] a nation than wars or economic depression.

See the words in bold? They are different each time. Yet, in the original sentence, they use the word "than" in this position. But that's wrong, because you cannot say:

* Disease can be as harmful to [...] a nation than wars [...]

For the sentece to be grammatically correct, you would need to construct it like this:

To the well-being and progress of a nation, disease can be as harmful as, if not more so than, wars or economic depressions.

That breaks down into the following two sentences:

[...] disease can be as harmful as wars or economic depressions
if disease is not more harmful than wars or economic depressions

As I said, this error is a very common one, and you will see it a lot in newspapers and magazines.