The correct answer is: D. none of the above
Conventional hydrogen cooling is not able to increase the rating of a single unit to 50 MW, 100 MW, or 200 MW. In fact, conventional hydrogen cooling is not able to increase the rating of a single unit at all.
Hydrogen cooling is a type of cooling that uses hydrogen gas to cool the components of a power plant. Hydrogen is a very good conductor of heat, so it can be used to remove heat from the components of a power plant very quickly. This can help to improve the efficiency of the power plant and reduce its emissions.
However, hydrogen cooling is not without its challenges. Hydrogen is a very flammable gas, so it must be handled with care. Additionally, hydrogen is a very light gas, so it can leak easily. This means that hydrogen cooling systems must be designed to be very leak-tight.
Despite these challenges, hydrogen cooling is a promising technology that has the potential to improve the efficiency and reduce the emissions of power plants. However, it is not yet a mature technology, and more research is needed to develop it further.