Translation:You must help me!
一定 means in general term " be sure ". But the correct answer was You HAVE to help me... "You Have to" in general term means 不得不。
In English, you must/have to help me mean the same thing and are interchangeable.
Do we actually need 要 here, since "You must help me" & "You must WANT to help me" do mean differently to me.