If the unknown side is a leg, the hypotenuse must be 26 (the longest of the other two sides). Then the Pythagorean theorem tells you
... 26² = 24² + y²
... y = √(26² -24²) = √100 = 10
The appropriate choice is
... 10
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You may recognize that this triangle is double the often-used Pythagorean triple (5, 12, 13).