To estimate the number 55, the option A is not correct because 7.50 squared equals 56.25. That is already higher than the number 55. Option B is correct because the average of 7.4 and 7.5 equals 7.45. The square of 7.45 equals 55.5025 which is approximately 55.
Option C is not correct because the squares of numbers between 7.5 and 7.6 is already greater than the number 55.
Option D is also not correct because the squares of the numbers between 7.4 and 7.5 exceeds the number 55.
The correct option is B