Which of the following is expected to form a colourless complex?

Which of the following ions has the least magnetic moment?

Which one of the following ions has the same number of unpaired electrons as those present in $V^{3+}$ ion?

Although Zirconium $(Zr)$ belongs to the $4d$-transition series and Hafnium $(Hf)$ belongs to the $5d$-transition series, they show similar physical and chemical properties because . . . . . . .

Which of the following ions will exhibit colour in aqueous solution?

Which of the following substances will have the highest magnetic moment for its transition metal ion?