Ruling out the alternatives helps confirm the answer. The First Plan (1951-56) and Second Plan (1956-61) predate the introduction of high-yielding dwarf wheat and rice varieties in India, so agricultural transformation had not yet begun. The Fourth Plan (1969-74) came after the technology was already established and expanding. It is only during the Third Plan (1961-66) that HYV seeds, chemical fertilizers and assured irrigation were first packaged together as a national strategy, marking the start of the Green Revolution. Hence, the correct period is the Third Five Year Plan.