What is the atomic number of the first post-actinoid element?

Identify the correct decreasing order of atomic size from the following.

The electronic configuration $3d^{10}4s^0$ is exhibited by which of the following ions?

Which of the following ions has electronic configuration $[Ar]3d^6$ ?
(At. nos. $Mn = 25, Fe = 26, Co = 27, Ni = 28$)

The element which shows only one oxidation state other than its elemental form is:

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