Thanks for posting your question here Aris
Before I answer your query, I want you look at the following sentence.
a. There were many apples
in the basket.
b. This is a car
Can you see how the singularity/plurality of the verb in the above sentences is determined?
“There”, “This” etc. act as “Placeholder pronouns” and do not directly act as subjects. They simply point to a particular object somewhere. (Aha! There it is!)
Verb is singular/plural depending on what the verb is linked to.
As you can see, in example (a) the verb is linked to “apples
” and hence we use the plural “were
Similarly in example (b) the verb is linked to “car
” and so we use the singular “is
Now let us come back to the question on hand. I hope you might have noticed already that we use the plural “are
” because of the plural “populations
The clause “there are much larger populations of pesticide-degrading microbes in soils with a relatively long history of pesticide use than in soils” answers the question “What reason is suggested by the finding?” and “that” simply acts as a “connector” here.
To answer your second question, please provide your detailed analysis on what you understood about the sentence and what errors you found in each option so that we can correctly identify where you are going wrong. This will minimize the number of queries and save time for both of us.
Thanks for cooperating with us.