To determine which right triangle has an unknown leg measure of
Since you're asking specifically about a leg measure, it can be either one of the legs (the two shorter sides of the triangle).
If one of the legs is
To identify a specific triangle, you may need additional information, like the length of the other leg or the length of the hypotenuse, or context regarding the triangle (such as whether it is part of a specific geometry problem).
Without any further context or choices provided for specific triangles, it's impossible to determine which right triangle has a leg of