Since the mode simply identifies whichever value shows up most often in a list, the quickest way to spot it is to scan the data and note how many times each distinct number appears.
Going through the set 2, 3, 3, 5, 7 one number at a time, 2 shows up a single time, 3 shows up twice in a row, and both 5 and 7 show up only once each.
Because every other value in this list is a singleton and only 3 repeats, there's no ambiguity or tie to resolve, this data set has exactly one mode.
So the correct answer is 3.