Which ion has maximum magnetic moment

Magnetic moment $2.84\, B.M.$ is given by (At. nos. $Ni = 28, Ti = 22, $$Cr = 24, Co = 27$)

Which of the following statements are correct?

$(A)$ The $M ^{3+} / M ^{2+}$ reduction potential for iron is greater than manganese

$(B)$ The higher oxidation states of first row $d$ block elements get stabilized by oxide ion.

$(C)$ Aqueous solution of $Cr ^{2+}$ can liberate hydrogen from dilute acid.

$(D)$ Magnetic moment of $V ^{2+}$ is observed between $4.4-5.2\,BM$

Choose the correct answer from the options given below:

Technetium, the element below manganese in the Periodic Table, would be expected to have high values for its 
$(I)$ melting point $(II)$ boiling point $(III)$ density

Why the transition elements are hard and have high melting and boiling points?

How would you account for the following:

$(i)$ of the $d^{4}$ species, $Cr ^{2+}$ is strongly reducing while manganese $(III)$ is strongly oxidising.

$(ii)$ Cobalt $(II)$ is stable in aqueous solution but in the presence of complexing reagents it is easily oxidised.

$(iii)$ The $d^{1}$ configuration is very unstable in ions.