This sentence explains that the reason why the generals of the army of the Potomac were singularly unsuccessful against Lee is discussed in a number of journal.

"Why the various Generals of the Army of the Potomac were singularly unsuccessful against Lee" is called a substantive clause. I.e, it is a type of clause or phrase that may contain a gerund or an infinitive and is considered singular, even if it may contain plural words. In this case, it is supposed to be closely followed by its verb showing the action.

Option A - incorrect
the addition of the comma to the sentence makes it seem like a run-on sentence. It makes the sentence seem like a fragment of a main sentence which incorrect.

Option B - incorrect
The answer choice is incorrect as it uses the wrong form of pronoun 'they' to represent a singular noun. The plural verb 'are' used is also wrong.

Option C - correct
Because of the absence of the comma, it explains clearly 'why various generals were unsuccessful against Lee is debated.' The verb is connected directly to the clause.

Option D - incorrect
This doesn't have a verb. 'debated' used works as a modifier but it doesn't show who it is modifying.

Option E - incorrect
This is also a fragment... 'Debating' functions as a modifier but it is not clear who it is modifying.