The vertical distance from S to T is 3.5 units
The horizontal distance from S to T is 12 units
So we have a right triangle forming with the legs of 3.5 and 12. The hypotenuse is x
Use the pythagorean theorem
a^2+b^2 = c^2
3.5^2 + 12^2 = x^2
12.25+144 = x^2
156.25 = x^2
x^2 = 156.25
x = sqrt(156.25)
x = 12.5
Answer is choice D) 12.5