Introduction:
Once-through boilers are a type of boilers that operate under specific pressure conditions. The given question asks about the pressure conditions at which these boilers operate. The correct answer is option 'C', which states that once-through boilers operate at both subcritical and supercritical pressures.

Explanation:
To understand why once-through boilers can operate at both subcritical and supercritical pressures, let's first define these terms:

1. Subcritical pressure: Subcritical pressure refers to the pressure below the critical point of a substance. In the context of steam power generation, subcritical pressure is below the critical point of water, which is around 22 MPa (megapascal) or 3200 psi (pound-force per square inch).

2. Supercritical pressure: Supercritical pressure refers to the pressure above the critical point of a substance. In the case of steam power generation, supercritical pressure is above the critical point of water, which is around 22 MPa or 3200 psi.

Operating Range of Once-Through Boilers:
Once-through boilers are known for their ability to operate over a wide range of pressure conditions. This versatility allows them to be used in various industrial applications. The operational range of once-through boilers includes:

1. Subcritical Pressure Operation:
- Once-through boilers can operate at subcritical pressures, which are below the critical point of water.
- In subcritical operation, the water inside the boiler remains in a liquid phase throughout the process.
- Subcritical operation is commonly used in conventional coal-fired power plants.

2. Supercritical Pressure Operation:
- Once-through boilers can also operate at supercritical pressures, which are above the critical point of water.
- In supercritical operation, the water inside the boiler exists as a single-phase fluid, with properties of both liquid and steam.
- Supercritical operation offers several advantages, including higher thermal efficiency and reduced emissions.
- Supercritical once-through boilers are commonly used in modern power plants for their improved efficiency.

Conclusion:
Once-through boilers are capable of operating at both subcritical and supercritical pressures. This flexibility allows them to be used in a wide range of industrial applications, including conventional coal-fired power plants and modern supercritical power plants. The ability to operate at different pressure conditions makes once-through boilers a versatile choice for steam power generation.