Start at the top and do not skip the middle. The President is the Supreme Commander of the Armed Forces. Day-to-day political control sits with the Union government; the Ministry of Defence is the civilian department that owns policy, money and the paperwork that ties the services to Cabinet.
Under MoD, the Chief of Defence Staff (CDS) heads the Department of Military Affairs (DMA). The three service chiefs - Army, Navy, Air Force - still run their own services. CDS is the joint adviser, not a replacement for the Cabinet and not the Supreme Commander. That last line alone clears a surprising number of wrong options.
Also park these names next to CDS: Integrated Defence Staff, plus specialised tri-service bodies such as the Defence Cyber Agency, Defence Space Agency and the Armed Forces Special Operations Division. They are joint constructs. Writing "Army unit with a fancy name" is how people lose marks.
Prelims angle: Supreme Commander = President. CDS ≠ Supreme Commander.