Translation:This shop is always busy.
"Crowded", as in there are a lot of people relative to the amount of space there is, would be 混んでる (こんでる). As Katsudamn mentioned, にぎやか has more of a connotation of being "lively" or "full of activity". For example, an elevator full of people would be described as 混んでる but not (necessarily) にぎやか.
In this exercise, the shop is being described as having a lot going on (にぎやか). It might also have a lot of people in it (混んでる) or not but the word choice of the sentence doesn't tell you either way.