This is a square means ithas all sides equal
Let say side is x
And diagonal length is given 5
U know something is called pythagorean thoeram
Which says relationship between sides of a right triangle
Square has right triangle each side
So hypotenuse^2=lenghth^2+base^2
Here hypotenuse is diagonal which is given 5
So 5^2=x^2+x^2
25=2x^2
x^2=12.5
x=√12.5=2.5√2
Answer is A
