It would be 7. It would be this because 7 time 7 is 49 and 8 times 8 would be 64. Subtract 53 and 49, and you would get 4. Then, you do 64 and 53, which would be 11. You could see that 49 is closer to 53, which means 7 would be closer to the square root of 53.
Another way you could do it is actually square rooting it. It would be 7.28.... Now if you round it, it would be about 7.