The correct answer is (a) Endohedral Fullerene.
Endohedral fullerenes are a type of fullerene that contains atoms of other elements inside its carbon cage. They are extremely rare and difficult to produce, which makes them very expensive. The most expensive endohedral fullerene is $C_{60}@^{136}Xe$, which contains a single xenon atom inside a carbon cage. It is estimated to cost over $100 million per gram.
Californium 252 is a radioactive element that is used in medical research and industrial applications. It is not a metal, and it is not particularly expensive. Tritium is a radioactive isotope of hydrogen that is used in glow-in-the-dark watches and other applications. It is also not a metal, and it is not particularly expensive. Rhodium is a metal that is used in catalytic converters and other applications. It is more expensive than gold, but it is not the most expensive metal in the world.
Here is a brief description of each option:
- Endohedral fullerenes are a type of fullerene that contains atoms of other elements inside its carbon cage. They are extremely rare and difficult to produce, which makes them very expensive. The most expensive endohedral fullerene is $C_{60}@^{136}Xe$, which contains a single xenon atom inside a carbon cage. It is estimated to cost over $100 million per gram.
- Californium 252 is a radioactive element that is used in medical research and industrial applications. It is not a metal, and it is not particularly expensive.
- Tritium is a radioactive isotope of hydrogen that is used in glow-in-the-dark watches and other applications. It is also not a metal, and it is not particularly expensive.
- Rhodium is a metal that is used in catalytic converters and other applications. It is more expensive than gold, but it is not the most expensive metal in the world.