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

The correct option is: All State Government employees joining the services after the date of notification by the respective State Governments.

Detailed Explanation:

Let us analyze each of the options provided to understand who is eligible to join the National Pension System (NPS):

1. Eligibility for State Government Employees:
Following the introduction of the NPS by the Central Government, almost all State Governments (except Sikkim initially) adopted the National Pension System for their employees. State Government employees joining the service after the specific notification date issued by their respective State Governments are covered under the NPS. Therefore, this statement is correct.

2. Analysis of Other Options:

• "Resident Indian citizens only" – Incorrect. Non-Resident Indians (NRIs) who are Indian citizens between 18 and 70 years of age are also eligible to open an NPS account (OCIs and PIOs are not eligible).

• "Persons of age from 21 to 55 only" – Incorrect. Any Indian citizen (resident or non-resident) aged between 18 and 70 years can join the NPS.

• "All Central Government employees including those of Armed Forces joining the services on or after 1st April 2004" – Incorrect. Armed Forces personnel are explicitly excluded from the NPS scheme, although Central Government employees joining on or after January 1, 2004 (except Armed Forces) are covered under NPS.

Conclusion:
Among the given options, the correct statement regarding eligibility to join the National Pension System is "All State Government employees joining the services after the date of notification by the respective State Governments".