Statement A and D are correct.
John enters the supermarket intending to steal (fulfilling the intent requirement of s.9(1)(a)). By entering the manager's office, a part of the building from which he may be implicitly excluded, he becomes a trespasser. Therefore, Statement A - that John is guilty of burglary under s.9(1)(a) - is correct.
Statement D is also correct, as once inside the manager's office John steals the £20 note, fulfilling the requirements of s.9(1)(b) of having entered as a trespasser and stealing something.
Statements B and C are not correct as the facts do not support robbery