Yes. But you have to qualify entrance for M.Sc. in respective stream, i.e. Entance exam for M.Sc. in botany or zoology. BHU offers hons. subject which you have to select among BZC. The selected subject in B.Sc. would be your subject in M.Sc. too.

There is one more gateway. If, suppose you have selected Zoology as Hons. subject in BSc but want to do MSc in other subject, say botany, then there is a provision for common Msc entrance test. In which all subjects- B, Z and C will be asked. Your score will decide your future.