A closed system is cooled reversibly from 100°C to 50°C. If no work is done on the system

Answer: Option B

Since here heat is taken from the system $$Q=-ve$$
From second law of thermodynamics $$TdS\ge \delta Q$$
Since given reversible process $$TdS=\delta Q$$
$$\Rightarrow dS=-ve$$
From first law of thermodynamics
$$\begin{array}{rl}& \delta Q=dU\\ & \Rightarrow dU=-ve\end{array}$$