Beryllium has higher ionisation enthalpy than boron. This can be explained as,

A. beryllium has higher size than boron hence its ionisation enthalpy is higher

B. penetration of $$2p$$ - electrons to the nucleus is more than the $$2s$$ - electrons

C. it is easier to remove electron from $$2p$$ - orbital as compared to $$2s$$ - orbital due to more penetration of $$s$$ - electrons  

D. ionisation energy increases in a period