The enrollment in professional courses has dropped considerable due to overall slump in the economy and poor employment prospects for recent college graduates.

Explanation

"The enrollment in professional courses has dropped considerably due to the overall slump in the economy and poor employment prospects for recent college graduates."The error is in the use of "considerable." The correct term here is "considerably," which is the adverb form with suffix -ly. It should modify the verb "dropped" to indicate the extent of the drop. Note that one important step before choosing the correct answer is to identify which word is being modified by the modifier. Here the drop rate is considerable not the enrollment and dropping being a verb makes considerable an adverb and is changed to considerably.So, the corrected sentence is: "The enrollment in professional courses has dropped considerably due to the overall slump in the economy and poor employment prospects for recent college graduates."