Step 1: Understand the question.
We need the imaginary line that passes almost through the middle of India.
Step 2: Identify it.
The Tropic of Cancer, at about $23^\circ 30'$ North, runs nearly through the centre of India.
Step 3: Its effect.
It divides India into tropical and subtropical zones and passes through eight Indian states.
Step 4: Rule out others.
The Equator is at $0^\circ$. The Tropic of Capricorn is in the Southern Hemisphere. The Prime Meridian is a longitude through Greenwich.
Step 5: Conclude.
The line through the middle of India is the Tropic of Cancer.
Answer: Tropic of Cancer