Good Question!
So since we know that a square is exactly the same all the way around we can assume that both diagonals in the square are the same. This means that they form a right triangle in the middle with two legs of 3.75!
So to find the side of the triangle we use a^2 + b^2 = c^2
3.75^2 + 3.75^2 = c^2
28.125 = c^2
√(28.125) = c
c = 5.3
Now finally we just square 5.3 again to find the area of the square and it's 28.1mm^2!
So the answer is the third one! hope this helps!