In English (or at least in my dialect) "marriage" usually means the ongoing relationship between spouses and "wedding" usually refers to the singular event of joining the two together. Since both are accepted as answers here, does γάμος carry all of that weight by itself, or are there other words as well?
When using the hints you should always choose the top one. They are set up so that the correct word/phrase for each sentence is at the top. The others will rise to the top when needed.
However, in this case both "marriage' and "wedding" are correct, and depending on the context you could use either. For this sentence without context both are correct. For other sentences in the lesson, you'll notice that one is better than the other.