I think this is an OG question and the OA is E here.

(A) lack math skills to such a large degree as to make it difficult to absorb them into a city economy becoming
So x as to Y is the correct idiom
(B) lack math skills to a large enough degree that they will be difficult to absorb into a city's economy that becomes
"becomes" makes signifies that this hasn't happened yet, but actually it is happening at present. We need becoming for that.
(C) lack of math skills is so large as to be difficult to absorb them into a city's economy that becomes
"becomes" makes it seem as if this hasn't happened yet, but the point is that it is happening at present. We need becoming for that.
(D) are lacking so much in math skills as to be difficult to absorb into a city's economy becoming
PROBLEM: Again, the so X as to Y is the correct idiom, not so X as to be Y.[/color]
(E) are so lacking in math skills that it will be difficult to absorb them into a city economy becoming
So X as to Y is correct. Becoming is correctly used here and signifies that it is happening at present