(C)
Explanation:
[tex]t = 2\pi \sqrt{ \frac{l}{g} } [/tex]
If g is only 1/6 on another planet, then
[tex]t = 2\pi \sqrt{ \frac{l}{ \frac{g}{6} } } = 2\pi \sqrt{ \frac{6l}{g} } [/tex]
[tex] = \sqrt{6} \: (2\pi \sqrt{ \frac{l}{g} } ) = 2.4 \times t(on \: earth)[/tex]