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