Let's solve for the other leg length
a^2 + 21^2 = 42^2
a^2 + 441 = 1764
subtract 441 from both sides
a^2 = 1323
take the square root of both sides
a = sqrt.(1323)
a = 21 sqrt.(3)
The correct answer is C.) The length of the remaining leg is 21 sqrt.(3).
Hope this helps