Answer:
"To learn Portuguese" is a fragment for the following reason:
C. It is not a complete thought.
Step-by-step explanation:
Saying "to learn Portuguese" isolatedly sounds intelligible, but also vague, since it lacks other basic elements found in a sentence, such as a clear subject and a predicate. In "Zoe decided to move to Portugal after graduating from college. To learn Portuguese", punctuation makes the last part a fragment. If we drop the period after "from college", "to learn Portuguese" becomes a part of the first sentence and is no longer a fragment, as in:
Zoe decided to move to Portugal after graduating from college to learn Portuguese.
"To learn Portuguese" becomes a part of the sentence and conveys the idea of purpose.