# Lanthanoids are

$\begin{array}{1 1}(a)\;\text{14 elements in the sixth period (atomic no = 58 to 71) that are filling 4f sublevel}\\(b)\;\text{14 elements in the sixth period (atomic no = 90 to 103) that are filling 4f sublevel}\\(c)\;\text{14 elements in the seventh period (atomic no = 58 to 71) that are filling 4f sublevel}\\(d)\;\text{14 elements in the seventh period (atomic no = 90 to 103) that are filling 4f sublevel}\end{array}$

14 elements in the sixth period (atomic no. = 58 to 71) that are filling 4f sublevel
As sixth period can accommodate only 18 element in the table, 14 member of HF series (atomic number 58 ot 71) are separately accommodated in a horizontal row below the periodic table. These are called as lanthanides.
Hence (a) is the correct answer.