Answer:
Option-(C): Lanthanide series.
Explanation:
Lanthanide are those elements which can be referred to as those elements inside the periodic table which are still unsettled and needs there proper place inside the periodic table.
- The lanthanides series is also called as the F block elements, as they have there atomic number started from 57 through 71.
- As these elements are more hard to separate from one another due to too much hardship in the procedure followed by the professionals and that is why the elements are said to be rare in nature.
- As the elements are considered more hard to break and are considered as one of the toughest in the universe to curve.
