ANS:C - hydrogen

The commonly employed coolant for high-pressure turbogenerators is typically hydrogen. Hydrogen is favored for several reasons:

1. High Thermal Conductivity: Hydrogen has very high thermal conductivity, which helps in efficient heat transfer and cooling.
2. Low Density: Hydrogen has a low density, which reduces the windage losses and increases the power density of the machine.
3. Non-Corrosive: Hydrogen is non-corrosive, which helps in maintaining the integrity of the generator components over time.

While water and air can be used as coolants in certain applications, they are less commonly used in high-pressure turbogenerators due to factors such as corrosion risk, lower thermal conductivity, and potentially higher windage losses. Carbon dioxide may be used in some specific applications but is less common compared to hydrogen for high-pressure turbogenerators.