Total number of $d$ electrons present in an element with atomic number $78$ is:

Which of the following has the highest ionic radii?

Because of lanthanoid contraction,which of the following pairs of elements have nearly same atomic radii? (Numbers in the parenthesis are atomic numbers)

The transition elements have a characteristic electronic configuration which can be represented as:

The correct order of density for the $3d$ series elements is:

Which of the following is the common oxidation state for transition elements?