b
(b)In sum position \(T \propto \frac{1}{{\sqrt {{M_1} + {M_2}} }}\)and in difference position \(T \propto \frac{1}{{\sqrt {{M_1} - {M_2}} }}\)
\( \Rightarrow \frac{{{3^2}}}{{{T^2}}} = \frac{{2M - M}}{{2M + M}} \Rightarrow {T^2} = 9 \times 3\,{\sec ^2}\)
\(\therefore \;T = 3\sqrt 3\, \sec \)