Hi abhasjha,

Part of the proposed increase in state education spending is due to higher enrollment, since the number of students in public schools have grown steadily since the mid-1980's and, at nearly 47 million, are at a record high.

A. enrollment, since the number of students in public schools have grown steadily since the mid-1980's and, at nearly 47 million, are at
Wrong: subject-verb agreement error. "The number" requires a singular verb, "A number" requires a plural verb.

B. Enrollment, with a number of students in public schools growing steadily since the mid-1980's and, at nearly 47 million, reaching.
Wrong: since is a conjunction that expresses reason. The reason of increase in education spendig has already been given "higher enrollment". The author is trying to add some more informations on the matter. "with" is a weak subordinator and does not convey the author's idea properly. Moreover growing is ambiguous, given that -ing modifiers if not placed after a comma modify the preceding noun, "growing" may modify "schools", leading to a logical error.

C. Enrollment: since students in public schools have grown steadily in number since the mid-1980's and, at nearly 47 million, have reached.
Wrong: It seems to convey that students have grown in height. Not the intended meaning.

D. Enrollment: the number of students in public schools has grown steadily since the mid-1980's and, at nearly 47 million, has reached.
Correct: everything has been fixed.

E. Enrollment: students in public schools have grown steadily in number since the mid-1980's and , at nearly 47million, are at
Wrong: Same error as C.

-TathastuGMAT

Hope it helps