Could someone i'm not sure how i'm supposed to solve this! (math)

We can solve this one by using the Pythagorean Theorem
Which is a^2+b^2=c^2

C is the hypotenuse, also known as segment AD.
To find the hypotenuse we do a^2, which is either of the two sides plus b^2 which is the other side

a^2 in this case is 4 units
and b^2 in this case is 4 units

4^2+3^2=25
we then take the sqrt if 25 which is 5

So segment AD=5

C is your correct answer