The correct answer is B. I'll explain again. To make myself clear.
Explanation:
A sentence fragment refers to a set of words that seem to go together but different to a complete sentence do not express a complete idea, usually because essential elements such as subjects, verb or objects are missing or because they need from a complete sentence to stand. This occurs in the case of " I'll explain again. To make myself clear. " because in this case "I'll explain again" acts as an independent complete sentence, but "To make myself clear" that is not linked to the first sentence is just a fragment because it does not express and complete idea and needs from another sentence, which makes it a sentence fragment and not a complete sentence.