The correct option is C under
Options A and C: 'Under' means 'lower in position'. It also means 'controlled or guided by'. It completes the sentence in meaning, as opposed to 'below'.
The preposition 'below' has a similar meaning as it refers to a lower position. But when you're referring to someone who is working in a position lower than another (General Manager), then you use 'under' and not 'below'.
(Also, in this context, 'below' refers to a physical level, as if he works below the floor where the General Manager works, but this isn't part of the sentence.)
Hence option A is incorrect and C is correct
Option B: 'Above' refers to 'a higher level or layer'. In this context, it refers to a physical layer, rather than a higher position. The meaning thus formed is ambiguous. Hence option B is incorrect.
Option D: Similarly, 'on' refers to the top of a surface or layer. It does not refer to the General Manager, hence this preposition isn't suitable and option D is incorrect.