When you post an SC, please underline the tested portion.

ardz24 wrote:Many entomologists say that campaigns to eradicate the fire ant in the United States have failed because the chemicals that were used were effective only in wiping out the ant's natural enemies, which made it easier for them to spread.

A. which made it easier for them
B. which makes it easier for it
C. thus making it easier for them
D. thus making it easier for the ant
E. thereby, it was made easier for the ant

A: the ant's natural enemies, which made it easier for them to spread
Here, both which and them seem to refer to enemies, conveying the following nonsensical meaning:
The ant's natural enemies made it easier for the ant's natural enemies to spread.
Eliminate A.

In B, which makes (which + SINGULAR VERB) lacks a clear singular referent.
Eliminate B.

C: The chemicals were effective only in wiping out the ant's natural enemies, thus making it easier for them to spread.
Here, them seems to refer to enemies -- the nearest preceding plural noun -- conveying the following meaning:
The chemicals effective in WIPING OUT the enemies were making it easier for the enemies TO SPREAD.
This meaning is nonsensical:
Enemies that are wiped out cannot spread.
Eliminate C.

E: The chemicals were effective only in wiping out the ant's natural enemies, thereby, it was made easier for the ant to spread.
Here, a comma incorrectly serves to connect the two complete sentences in red.
If a comma can be replaced with a period, the answer choice is wrong.
Eliminate E.

The correct answer is D.