The correct answer is uranium. Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-white metal that occurs naturally in uranium-bearing minerals. Uranium is a radioactive element that decays into thorium and then into lead. It is the heaviest naturally occurring element.
Mercury is a chemical element with the symbol Hg and atomic number 80. It is a silvery-white, heavy, and toxic liquid metal. Mercury is a member of the transition metals and is one of the few metals that is liquid at standard temperature and pressure. It is the only metal that is liquid at room temperature and pressure.
Polonium is a chemical element with the symbol Po and atomic number 84. It is a radioactive element that occurs naturally in uranium-bearing minerals. Polonium is a member of the metalloids and is a silvery-white, soft, and highly radioactive metal.
Thorium is a chemical element with the symbol Th and atomic number 90. It is a silvery-white, radioactive metal that occurs naturally in thorium-bearing minerals. Thorium is a member of the actinides and is a relatively stable element. It is the second most abundant naturally occurring radioactive element after uranium.
Uranium is the heaviest naturally occurring element because it has the highest atomic number. The atomic number is the number of protons in the nucleus of an atom. Uranium has 92 protons, while mercury has 80 protons, polonium has 84 protons, and thorium has 90 protons.