enthalpy increases along the period but decreases down the group.

The IE of an element increases as one moves across a period in the periodic table because the

electrons are held tighter by the higher effective nuclear charge.

The ionization energy of the elements decreases as one moves down the group because the electrons are held in lower-energy orbitals, away from the nucleus and therefore, are less tightly bound.

$A r$ has higher IE because it is a noble gas and $B a$ has the lowest IE as it is in 6 periods and more

metallic.

Hence, the correct option is $\mathrm{C}$