(C) Tertiary
A doctor in a government hospital provides a service (healthcare). Service activities fall under the tertiary sector.
The question tests two concepts: (1) doctors provide services → tertiary sector; (2) government hospital → public sector. The question asks about the primary/secondary/tertiary classification, not public/private. Since healthcare is a service activity, it belongs to the tertiary sector. Do not confuse "government" with "public sector" here — that is a different classification axis entirely.