Choice C is correct. Since the triangle is an isosceles right triangle, the two sides that form the right angle must be the same length. Let be the length, in inches, of each of those sides. The Pythagorean theorem states that in a right triangle, , where is the length of the hypotenuse and and are the lengths of the other two sides. Substituting for , for , and for in this equation yields , or . Dividing each side of this equation by yields , or . Taking the square root of each side of this equation yields two solutions: and . The value of must be positive because it represents a side length. Therefore, , or . The perimeter, in inches, of the triangle is , or . Substituting for in this expression gives a perimeter, in inches, of , or .
Choice A is incorrect. This is the length, in inches, of each of the congruent sides of the triangle, not the perimeter, in inches, of the triangle.
Choice B is incorrect. This is the sum of the lengths, in inches, of the congruent sides of the triangle, not the perimeter, in inches, of the triangle.
Choice D is incorrect and may result from conceptual or calculation errors.