Which pair of elements from the following has half-filled $d$-orbitals in their observed electronic configuration?

Arrange the following ions in the increasing order of their magnetic moments: $(i) \, V^{4+}$,$(ii) \, Mn^{4+}$,$(iii) \, Fe^{3+}$,$(iv) \, Ni^{2+}$. $[V = 23, Mn = 25, Fe = 26, Ni = 28]$

Which of the following has the most unpaired $d$-electrons?

Which of the following factors may be regarded as the main cause of lanthanide contraction?

The stability of the oxidation state in lanthanoid elements depends on which of the following?

Assertion : Cuprous ion $(Cu^+)$ has unpaired electrons while cupric ion $(Cu^{2+})$ does not.
Reason : Cuprous ion $(Cu^+)$ is colourless whereas cupric ion $(Cu^{2+})$ is blue in the aqueous solution.