Which of the following ions is paramagnetic?

Variable valency,in general,is exhibited by

How many unpaired electrons are there in $Ni^{2+}$?

Among the following pairs of ions,the lower oxidation state in aqueous solution is more stable than the other in:

Which of the following elements show $+7$ oxidation states?

Match the metal ions given in Column-$I$ with the spin magnetic moments of the ions given in Column-$II$ and assign the correct code.

Column-$I$	Column-$II$
$(a) \ Co^{3+}$	$(i) \ \sqrt{8} \ B.M.$
$(b) \ Cr^{3+}$	$(ii) \ \sqrt{35} \ B.M.$
$(c) \ Fe^{3+}$	$(iii) \ \sqrt{3} \ B.M.$
$(d) \ Ni^{2+}$	$(iv) \ \sqrt{24} \ B.M.$
	$(v) \ \sqrt{15} \ B.M.$

$(a) \quad (b) \quad (c) \quad (d)$