Explanation:

1. Stage 1: Use natural uranium to fuel pressurized heavy water reactors (PHWRs). The byproduct, Pu-239, of these reactors are key for Stage 2.
2. Stage 2: Develop fast breeder reactors (FBRs) to produce excess Pu-239 which will then be convert Th-232 to fissile U-233.
3. Stage 3: Build thorium-based reactors that can be refueled using India's thorium reserves, which are converted to U-233 inside the reactor.

Hence option 4th is correct.