Step 1: Rule out by tradition.
Each option names a whole education tradition, so check which one actually uses sanskaras (rites of passage) as part of schooling.
Step 2: Match the term to its source.
"Sanskar" is a Sanskrit, Vedic-tradition word for a purifying rite performed at a life stage. Upanayan is one of these 16 rites, performed before a child began formal study under a guru.
Step 3: Confirm against the alternatives.
Jain, Buddhist, and Islamic schooling traditions each have their own entry customs (initiation into a sangha, admission to a madrasa) but none is called a "sanskar" in this sense.
Final Answer:
The term and the practice both point to the Vedic system.