We need to use the Pythagorean theorem
[tex]c^2=a^2+b^2[/tex]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
[tex]a^2+b^2=7^2+6^2=85[/tex][tex]c^2=9^2=81[/tex][tex]81\ne85[/tex]it is not a right triangle
for 12m, 10m, 8m
[tex]a^2+b^2=10^2+8^2=164[/tex][tex]c^2=12^2=144[/tex][tex]144\ne164[/tex]it is not a right triangle
for 16m,10 m,12m
[tex]a^2+b^2=10^2+12^2=244[/tex][tex]c^2=16^2=256[/tex][tex]256\ne244[/tex]it is not a right triangle
for 20m, 21m, 29m
[tex]a^2+b^2=20^2+21^2=841[/tex][tex]c^2=29^2=841[/tex][tex]841=841[/tex]It is a right triangle
the correct answer is D.