A. 'The number of' is singular. There 'have grown' is incorrect.
B. with a number of students .. growing implies that the students are growing. Also, adverbial
clause "with....." incorrectly modifies " spending is"... does not make sense.
C. This is wrong, because you can't follow a colon with since/although/etc. + clause. (if the
second part starts with something like 'since', it should be introduced with a comma, not a colon.)
'grown steadily in number' is also awkward.
D. Correct.
E. 'At 47 million' seems to modify students and not number of students. Incorrect.