Correct Option: D

Thorium is a naturally occurring radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 by the Norwegian mineralogist Morten ThraneEsmark and identified by the Swedish chemist JonsJakob Berzelius and named after Thor, the Norse god of thunder. In nature, virtually all thorium is found as thorium-232, which undergoes alpha decay with a half-life of about 14.05 billion years. Other isotopes of thorium are short-lived intermediates in the decay chains of higher elements, and only found in trace amounts. Thorium is estimated to be about three to four times more abundant than uranium in the Earth’s crust, and is chiefly refined from monazite sands as a by-product of extracting rare earth metals. Thorium was once commonly used as the light source in gas mantles and as an alloying material, but these applications have declined due to concerns about its radioactivity.