Correct Option (C)
The National Pension System (NPS) was implemented by the Central Government from January 1, 2004 (excluding the Armed Forces). Subsequently, various State Governments adopted the NPS framework and implemented it for their employees from different dates. Therefore, all State Government employees joining services after the notification date by their respective State Governments are eligible to join NPS.
Incorrect Options:
A. The NPS is open to any citizen of India, including both resident and non-resident Indians (NRIs), provided they meet other eligibility criteria. Thus, the restriction to "Resident Indian citizens only" is incorrect.
B. The age eligibility for joining NPS is between 18 and 60 years. Individuals cannot make contributions to NPS accounts after attaining 60 years of age. Therefore, the age range of "21 to 55 only" is incorrect.
D. While the NPS was introduced for Central Government employees joining services on or after January 1, 2004, it specifically excludes the Armed Forces. Therefore, the statement including "those of Armed Forces" is incorrect.