Which of the following cations in their respective oxidation states develops a colorless aqueous solution?

What may be the stable oxidation states of the transition elements with the following $d$ electron configurations in the ground state of their atoms: $3d^3$,$3d^5$,$3d^8$,and $3d^4$?

The atomic radii from $Cr$ to $Cu$ are almost identical because of:

The calculated magnetic moment (in Bohr magnetons) of $Cu^{2+}$ ion is:

Gadolinium (atomic number $= 64$) is a member of the $4f$ series. Its electronic configuration in the $+3$ oxidation state is $[Xe] 4f^7$. What is the ground state electronic configuration of gadolinium?

If the colours of salts of transition elements are due to the presence of unpaired electrons in the transition metal ions,which of the following ions will be colourless in aqueous solution? (The atomic numbers of $Ti$ and $Fe$ are $22$ and $26$ respectively.)