A

In a Carnot cycle, the first operation is an isothermal expansion.
The Carnot cycle consists of four processes:
1. **Isothermal Expansion** (A to B): The gas expands isothermally at a high temperature $T_H$. During this process, it absorbs heat $Q_H$ from the hot reservoir and does work on the surroundings.
2. **Adiabatic Expansion** (B to C): The gas continues to expand but without exchanging heat with the surroundings, thus its temperature decreases.
3. **Isothermal Compression** (C to D): The gas is compressed isothermally at a lower temperature $T_C$, releasing heat $Q_C$ to the cold reservoir.
4. **Adiabatic Compression** (D to A): The gas is compressed adiabatically, which raises its temperature back to $T_H$.
Therefore, the first operation of the Carnot cycle is indeed isothermal expansion.