The correct option is
C In
Option B: 'In' indicates inclusion within an abstract or immaterial thing, like 'service'.
'In the service' means 'to work for someone permanently'.
The preposition is relevant to the given sentence, and hence option B is correct.
Option A: 'With' means 'accompanied by'.
Option C: 'Under' means 'beneath the surface of'.
Option D: 'For' means 'with the object or purpose of.
These words do not form a complete meaningful sentence.
Hence options A, C and D are incorrect.