Who among the following can join the National Pension System (NPS)?

Resident Indian citizens only
Persons of age from 21 to 55 only
All State Government employees joining the services after the date of notification by the respective State Governments
All Central Government employees including those of Armed Forces joining the services on or after 1st April, 2004

Answer

All State Government employees joining the services after the date of notification by the respective State Governments

(a) Incorrect. NPS is not limited to resident Indians only; NRIs can also join NPS subject to RBI/FEMA norms.
(b) Incorrect. The age condition given is wrong. NPS was open to citizens in the 18–60 years bracket (not 21–55).
(c) Correct. State Government employees joining service after the date notified by the concerned State Government are covered under NPS.
(d) Incorrect. For Central Government employees, NPS applies from 1 January 2004, and Armed Forces were excluded from this mandatory coverage.

Therefore, only option (c) is correct.