Who functions as chairman, Chiefs of Staff Committee?

Who functions as chairman, Chiefs of Staff Committee?

Defence Minister
The longest serving Chief among the Chiefs of the three services
The Chief of Integrated Defence Staff
The Chief of the Army Staff
This question was previously asked in
UPSC CAPF – 2015
The correct option is B.
Prior to the creation of the Chief of Defence Staff (CDS) position as the permanent Chairman of the Chiefs of Staff Committee (COSC), the Chairman of the COSC was the longest-serving among the three Service Chiefs (Army, Navy, and Air Force). This question appears to be framed in the context of that earlier system, as the CDS is not listed as an option and the Chief of Integrated Defence Staff is a different role.
The Chiefs of Staff Committee (COSC) consists of the Chiefs of the Army Staff, the Naval Staff, and the Air Staff. With the creation of the Chief of Defence Staff (CDS) post in late 2019/early 2020, the CDS became the permanent Chairman of the COSC, serving as the Principal Military Advisor to the Defence Minister. Before the CDS, the chairmanship rotated based on seniority among the three service chiefs. The Chief of Integrated Defence Staff reports to the CDS.