The third ionisation enthalpy of Mn is distinctly higher than that of Fe because

AMn has a smaller atomic radius than Fe
Bremoval of the third electron from $Mn X^{2 +}$ ( $3 d^{5}$ ) disrupts a stable half-filled subshell, while for $Fe X^{2 +}$ ( $3 d^{6}$ ) it gives $3 d^{5}$
CMn has more unpaired electrons than Fe in its neutral atom
Dthe effective nuclear charge of Fe is lower than that of Mn

Answer: B

Mn X^{2 +}

3 d^{5}

, a half-filled and exchange-stabilised arrangement, so extra energy is needed to remove a third electron.

Fe X^{2 +}

3 d^{6}

, and losing one electron gives the stable

3 d^{5}

Fe X^{3 +}

, so the third ionisation enthalpy of Fe is lower.