I've already told you that the fourth and last part of the article____the most difficult to understand.
I choose "are"here because the sentence says two parts of the article, it should be plural. But the answer given is "is", what's the reason?
I've already told you that the fourth and last part of the article is / are the most difficult to understand.
Strictly speaking it's ambiguous, though singular "is" is the most salient interpretation, or "was" if a past time meaning is intended.
If the "last part" of the article is in fact also "the fourth part", then singular "is" (or "was") would be correct because we're talking about one and the same part -- a single entity, a single part of the article.
But if they are separate entities, i.e. a fourth part and a last part, then plural "are" (or "were") is correct.
The fact that singular "part" has been used leads me to think it's a single entity. If it was plural "parts", then there's no doubt at all that it should be plural "are" (or "were").
Thank you for enlightening me so much, Mr Matthews.
This sentence uses"the fourth and last part" instead of "the fourth and the last part", which may probably indicate the same part of the article. I guess that this could be the reason why the key given is "is". But as you mention, it should depend on a more detailed context to draw an more accurate conclusion.