1. Process identification

A slab of ice melts at $T=273\,\text{K}$ and at atmospheric pressure. This is a phase change from ice to water at constant temperature and constant external pressure.

2. Use the first law of thermodynamics

For the system (ice + water), $\Delta Q = \Delta U + W$ where $W$ is the work done by the system.

During melting, heat is absorbed as latent heat of fusion: $\Delta Q > 0$

3. Change in internal energy

Melting requires energy to weaken/break part of the intermolecular bonding structure of ice. Hence the internal energy increases: $\Delta U > 0$ So:

• Option A (internal energy unchanged) is false.
• Option D (internal energy decreases) is false.

4. Work done during melting

Ice has a larger volume than the same mass of water near $273\,\text{K}$. So when ice melts, the volume decreases: $\Delta V < 0$

At constant atmospheric pressure, work done by the system is $W = P\Delta V$ Since $\Delta V<0$, $W<0$ This means the system does negative work on the atmosphere, or equivalently, the atmosphere does positive work on the system.

Therefore:

• Option B is false.
• Option C is true.

5. Final conclusion

The correct option is: $\boxed{\text{C}}$