Answer
CIAD
Explanation
The Pythagorean theorem states:
where a and b are the legs and c is the hypotenuse of a right triangle.
Applying this theorem to triangle 1:
Then, the first letter is C.
Applying the theorem to triangle 2:
Then, the second letter is I.
Applying the theorem to triangle 3:
Then, the third letter is A.
Applying the theorem to triangle 4:
Then, the fourth letter is D.