(a)
$\because$ LCM of 3, 4, 6 is 12.
$\therefore\ \ \ \sqrt[3]{9} = 9^{1/3} = \left( 9^{4} \right)^{1/12} = (6561)^{1/12}$
$\sqrt[4]{11} = (11)^{1/4} = \left( 11^{3} \right)^{1/12} = (1331)^{1/12}$
$\sqrt[6]{17} = (17)^{1/6} = \left( 17^{2} \right)^{1/2} = (289)^{1/12}$
Hence, $\sqrt[3]{9}$ is the greatest number.