This question is about a frequently quoted fact from vexillology, the study of flags. Comparing how often each of the four motifs listed shows up across the roughly 190 or so national flags of the world points to the correct choice.
Because stars are used by countries across every continent and political tradition, far more widely than the crescent, an animal figure, or a striped pattern used as a standalone symbol, the star is recognised as the most common symbol placed on national flags.
Let's summarize:
The correct answer is Star.