Assertion and Reason type questions:

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 $2{\mathrm{s}}^2$ half –filled orbital than the electron from $2{\mathrm{p}}^1$ 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.