Option A: Wrong
The sentence formation is correct and understandable, comprehensive and logical to the audience. Nevertheless, it is not a summary version of what's required to deal with the writer's quest to find answers to the question in broader terms.

Option B: Wrong
The lost amount here is due to illiteracy and not directly to list individual output and tax revenues which are the calamities illiteracy has brought the United states and not the vice versa. The head word here, ''illiteracy'' has been replaced with the objects industrial output and to revenues

Option C: Right
This appears to be the most correct and logical in that, it expresses the views of the writer and strengthens his/her position more vividly. ''Illiteracy'' is the head-word here'', United states'' is the subject'' and industrial output and tax revenues'' is the object.

Option D: Wrong
Here, the reportage of the statement seeks to infer that cost is not negative to the united states which makes the argument further elaborate and debatable. The statement is over-specified and too lumped together to draw a concise argument.

Option E: Wrong
This statement would have been correct and appropriate but its arrangement is such that it only conveys the head-word towards the end of the statement which would not afford a layman or a lazy reader to understand quickly.

BTGmoderatorDC wrote:It has been estimated that the annual cost to the United States of illiteracy in lost industrial output and tax revenues is at least $20 billion a year.

(A) the annual cost to the United States of illiteracy in lost industrial output and tax revenues is at least $20 billion a year

(B) the annual cost of illiteracy to the United States is at least $20 billion a year because of lost industrial output and tax revenues

(C) illiteracy costs the United States at least $20 billion a year in lost industrial output and tax revenues

(D) $20 billion a year in lost industrial output and tax revenues is the annual cost to the United States of illiteracy

(E) lost industrial output and tax revenues cost the United States at least $20 billion a year because of illiteracy.

In A, B and D, annual and a year are redundant.
Eliminate A, B and D.

E: It has been estimated...because of illiteracy.
Here, a reader could construe that illiteracy CAUSES 20 billion a year to be estimated.
Not the intended meaning.
Eliminate E.

The correct answer is C.