Agricultural scientists have estimated that the annual loss by erosion

C is correct because it correctly uses, the idiom "loss of" stating that the land has lost "quality".

Also in this sentence it correctly uses the Cause + Results in + Effect Idiom from MGMAT pg.179 Q.11 A

The erosion is caused by rainfall which results in a loss of "quality of the land and the effect is that it approaches two million acres per year.

1) It's unclear whether "caused" modifies loss, erosion, or even land (although nonsensical).
2) "Annual" and "per year" are redundant.
3) "loss by erosion of arable land" has a potential and strange interpretation, suggesting that the loss approaches two million through the means of erosion.
Eliminate.


Since the verb "causes" is singular, our subject is only erosion instead of erosion and flood control. Constructed this way, the sentence suggests erosion OF rainfall and erosion OF flood control. You cannot erode either of those things - a definitive reason to eliminate this choice


This looks to be the best option. Erosion caused by X results in a loss approaching two million. It is okay for "approaching" to jump over land to modify "loss" because you have two essential modifiers describing the same thing. Or you could say that "approaching" is modifying the entire noun phrase "loss of arable land" - clear enough.


1) "Annual" and "per year" are redundant
2) This sentence reverses the position of modifiers by placing "approaching" ahead of "of arable land." Hard to say that this is wrong necessarily but it makes us pause and think "How does a loss APPROACH two million acres?" In this regard, preference goes to (C) over (D).


1) Placement of "annually" at the start introduces some problems. Presumably, it is meant to modify "two million acres" but those two phrases are placed so far apart that it's not clear. Annually" could also be interpreted to modify "caused," meaning that the loss of land is caused by erosion every year. In such case, we can no longer tell whether "two million acres" is the cumulative total loss or the loss per year.
2) As daagh already suggested, the structure of this sentence is "X is caused by Y caused by Z" - this structure is not necessarily wrong but diction preference goes to (C).

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