This problem is just try and error.
First we try number 2.5 since its the average of all the answers.
2.5^2=6.25
This is smaller than 7 so the answer is either 2.6 or 2.8
Now let's try 2.7
2.7^2=7.29
Which is bigger than 7 and that means the correct answer is 2.6 :)
