Which of the following pairs of elements does $NOT$ represent chemical twins?

The incorrect relationship in the following pairs in relation to ionisation enthalpies is $:$

Which of the following ions has the highest magnetic moment?

Which of the following pairs has similar ionic radii?

Those elements whose two outermost orbitals are incompletely filled with electrons are known as:

$A$ transition metal $M$ among $Sc, Ti, V, Cr, Mn$ and $Fe$ has the highest second ionisation enthalpy. The spin only magnetic moment value of $M^{+}$ ion is $. . . . . . . BM$ (Near integer)
(Given atomic number $Sc: 21, Ti: 22, V: 23, Cr: 24, Mn: 25, Fe: 26$)