Triangle has angles of 90°, 45° and 45° , therefore it's an isosceles right triangle, so the legs have the same length.
Let x be a leg.
[tex]sin(45^o)= \frac{opposite}{hypotenuse} = \frac{x}{24} \ \ \to \\  \\ x = 24*sin(45^o) = 24 *   \frac{ \sqrt{2} }{2}=12 \sqrt{2}  [/tex]
 
Answer: D.