I am assuming that the underlined portion is supposed to be equivalent to the enrollment of ?

The first thing I did was knock out A, B, and C. I can't provide you with a specific grammer rule as to why. It just seems that 'about' and 'equivalent/equal' don't fit together. It's either equivalent or it's about as many.

From that point, I nixed D because it inproperly relates 8 million to enrollment. The sentence could have been correct, if it compared parallel structures, i.e. "the enrollment of 8 million..... was as many as the enrollment of..." Or something like that.

That leaves E.

I hope this helped. Again, I apologize for not being able to cite specific grammer rules that would make it clearer for you.

option A is the correct answer because we don't say 'equivalent of' but 'equivalent to'. the option further gives the true usage of the word 'equivalent'.

in option B, it is incorrect because it is not right to say 'equivalent of' and this is what differentiate option A from B
in option C, it is improper to say 'about equal'. it is either something is equal or not.
in option D, 'about as many as' . This option suffers parallelism and it is does fit in to this question
option E also suffer the same fate as that of option D