It's B) 20
You would use the Pythagorean Theorem to find the length of the hypotenuse (c)
And the equation is a^2+b^2=c^2
Where a and b are the two other lengths of the triangle.
So in this case you'd substitute a and b with 12 and 16 to get:
12^2+16^2=c^2
Which simplifies to:
144+256=c^2
400=c^2
20=c
