Who functions as chairman, Chiefs of Staff Committee ?

The Chiefs of Staff Committee (COSC) was established in 1947 as the supreme professional body to advise the government on military matters. Historically, the chairmanship of the COSC was held in rotation by the senior-most (longest-serving) Chief among the three services [2]. This officer functioned as 'primus inter pares' (first among equals) but lacked executive power over other services [2]. In December 2019, the government created the post of Chief of Defence Staff (CDS), who now serves as the Permanent Chairman of the COSC [1]. However, in the absence of a CDS, the old system of appointing the senior-most service chief as the Chairman is utilized, as seen with General MM Naravane's appointment in 2021 [2]. Therefore, among the provided options, the longest-serving Chief of the three services is the correct functional description for the Chairman COSC.

Sources

1. [2] https://www.thehindu.com/news/national/lt-general-anil-chauhan-appointed-as-new-cds/article65946805.ece
2. [1] https://en.wikipedia.org/wiki/Chiefs_of_Staff_Committee_(India)