The correct answer is D. Thorium.
Monazite is a phosphate mineral that is found in sand and gravel deposits. It is a major source of thorium, a radioactive element that is used in nuclear power plants. Monazite also contains other elements, such as lanthanum, cerium, and neodymium, which are used in a variety of applications, including electronics and lighting.
Titanium is a metal that is used in a variety of applications, including aerospace, construction, and automotive. Zirconium is a metal that is used in a variety of applications, including nuclear power, jewelry, and ceramics. Iron is a metal that is used in a variety of applications, including construction, transportation, and manufacturing.
Thorium is a radioactive element that is used in nuclear power plants. It is also used in a variety of other applications, including lighting, electronics, and medical imaging.