A square has rotational symmetry of order ________.

In a square

Fig. 1 shows original form of a square

when we rotate it by $90°$ we get same form of square which we have seen in Fig.1. (fig. 2)

again we rotate it by $90°$ we get same form of square which we have seen in Fig.1. (fig. 3)

again we rotate it by $90°$ we get same form of square which we have seen in Fig.1. (fig. 4)

again we rotate it by $90°$ we get same form of square which we have seen in Fig.1. (fig. 5)

Therefore, it is clear that when we rotate the image by $90°$ we get the same form of the image as the original image.

So $\frac{360°}{90°} = 4$

Hence, a square has rotational symmetry of order four.