Correct Answer: Rare earth elements

Solution : The correct answer is Rare earth elements.

Lanthanides are sometimes known as rare earth elements. These are a set of 15 chemical elements in the periodic table with atomic numbers ranging from 57 (lanthanum) to 71 (lutetium). They are dubbed "rare earth" because they were initially discovered in unusual minerals, however, they are not uncommon in the Earth's crust.