The main difference between a nuclear power plant and thermal power plant is:
A. Nuclear power plant is larger and newer
B. Other plants produce pollution-free power
C. Heat produced in a nuclear reaction (fission) is used to make steam
D. Nuclear power plant converts heat into electricity

Hint
Whenever we talk about Nuclear and Thermal power the first thing that comes in our mind is power and electricity. Going to the root try to understand the basic terms what is a nuclear reaction and what does thermal energy means how it is produced. Nuclear power plant is often called a nuclear power station. In thermal power plants heat is Produced by burning coal. Try to come up with an answer after reading the hint.

Complete step by step solution
Nuclear Power Plant
A nuclear power plant or nuclear power station is a thermal power station in which the heat source is a nuclear reactor. As in all terms of thermal power stations the heat is used to generate steam which runs a steam turbine connected to an electric generator to produce electricity.
Nuclear plants are usually considered to be base load stations since fuel is a small part of the cost of production It means less fuel is required in a Nuclear Power plant to start nuclear reaction (fission mainly).
Advantages:
(i) One of the most low-carbon energy sources.
(ii) It also has one of the smallest carbon footprints.
(iii) It's one of the answers to the energy gap.
(iv) It is essential to our response to climate change and greenhouse gas emissions.
(v) Reliable and cost-effective.
Thermal Power Plant
A thermal power station is a power plant in which heat energy is converted to electric energy. In most of the places in the world the turbine is run by steam. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator.
Advantages:
Fuel used in this plant is less in cost.
Smaller space is required.
Economical in initial cost compared to Nuclear plants and running cost is less.
Thermal plants can be placed near load not like other types of plants. Thus transfer of power losses can be minimized.
After analysing all above information;
Clearly option (C) is correct.

Note
A nuclear plant transforms(converts) fission or "burning" of non-renewable fuel into electricity. By means of the fuels used, a thermal plant is much more inefficient and has environmental side effects. A nuclear power plant is a thermal power plant. Nuclear fission replaces fossil fuel as the heat source.