To find the area of a square, you can use the formula:
\[ \text{Area} = \text{side} \times \text{side} \]
In this case, the side length of the square is 30 yards. Therefore, the area of the baseball infield is:
\[ \text{Area} = 30 , \text{yards} \times 30 , \text{yards} = 900 , \text{square yards} \]
So the correct response is:
900 square yards.