We need to use the Pythagorean theorem

c is always the bigger side, if the sum of the square of the other sides is c^2 then we have a right triangle
for 9m , 7m, 6m



it is not a right triangle
for 12m, 10m, 8m



it is not a right triangle
for 16m,10 m,12m



it is not a right triangle
for 20m, 21m, 29m



It is a right triangle
the correct answer is D.