Answer:
comma splice
Explanation:
two possible revisions:
Mary Ellen was not following the directions. Therefore, she did not write an appropriate paper.
Mary Ellen was not following the directions; therefore, she did not write an appropriate paper.
The ONLY time that a comma can be used to connect two independent clauses is when it is paired with a coordinating conjunction. The word "therefore" is <em>not</em> a coordinating conjunction; it is a conjunctive adverb (which requires the punctuation as shown in the two revisions above.)
