Three blocks A, B and C are suspended as shown in figure. Mass of each of blocks A and B is m. If the system is in equilibrium, and mass of C is M, then : 

M < 2 m

Consider equation : \(2 T \cos {\theta} = Mg \text{ and} T = mg\)

\(\cos {\theta} = \frac{Mg}{2 mg} = \frac{M}{2 m}\)

\(\Rightarrow \cos {\theta} < 1\)

\(\frac{M}{2 m} < 1\)

\(\Rightarrow M < 2 m\)