Assertion (A)          : The first ionization energy of Be is greater than that of B.

Reason (R) : $2P$Electrons are more loosely held than $2S$ electrons.

Assertion: It is fact that first ionization energy of Be is greater than that of B because the electron has to be removed from the more stable  $2S^2$ half –filled  orbital than the electron from  $2p^1$ incompletely filled  orbital of B.

Reason: R  is correct ,and the correct explanation of A. Actually the ionisation energy depends on the  penetration  power of the orbitals , the order of penetration power of orbitals is $s>p>d>f$Therefore $2p$Electrons are more loosely held than $2s$electrons.

            $2p$ Electrons are more loosely held than $2s$ 2S electrons.