Which of the following is a colourless ion?

Among the following series of transition metal ions,the one in which all the metal ions have $3d^2$ electronic configuration (excluding the $3p^6$ core) is (Atomic number: $Ti=22, V=23, Cr=24, Mn=25$)

Ionisation potential values of $d-$block elements as compared to ionization potential values of $f-$block elements are

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

Which of the following elements does not exhibit variable valency?

Electronic configuration of $Gd$ $(Z = 64)$ is