The correct answer is uranium. Uranium is a naturally occurring radioactive chemical element with the symbol U and atomic number 92. It is a silvery-white metal that chemically resembles niobium and tantalum. Uranium is almost always found in combination with other elements in minerals. It is the heaviest naturally occurring element and the principal fissile component of nuclear weapons.
Mercury is a chemical element with the symbol Hg and atomic number 80. It is a silvery-white, heavy, dense, and toxic liquid metal. Chemically, mercury is one of the group 12 elements, and is classed as a transition metal.
Thorium is a chemical element with the symbol Th and atomic number 90. It is a silvery-white, radioactive, metallic chemical element in
the actinide series of the periodic table. Thorium is naturally occurring, and is found in many minerals, such as monazite and thorite.Protactinium is a chemical element with the symbol Pa and atomic number 91. It is a radioactive, silvery-white metal that is chemically similar to uranium. Protactinium is found in nature in very small amounts, and is produced artificially in nuclear reactors.