Agrarian Structure

Step 1: Understanding the Green Revolution.
The Green Revolution refers to the introduction of High Yielding Variety (HYV) seeds, modern irrigation methods, chemical fertilizers, and pesticides in India during the 1960s to increase agricultural production, especially food grains like wheat and rice.

Step 2: Impact during the first phase.
The first phase of the Green Revolution mainly focused on wheat production and was concentrated in states with good irrigation facilities and fertile soil. Punjab had well-developed irrigation systems and adopted modern farming techniques quickly, leading to a massive increase in wheat production.

Step 3: Analysis of the options.

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  (A) Madhya Pradesh: Incorrect. It did not benefit the most in the initial phase.
• 
  (B) Rajasthan: Incorrect. Limited irrigation reduced its early impact.
• 
  (C) Punjab: Correct. Punjab gained the maximum benefit during the first phase.
• 
  (D) Kerala: Incorrect. Kerala was not a major beneficiary of the Green Revolution initially.

Step 4: Conclusion.
Therefore, Punjab benefited the most from the Green Revolution in its first phase.

Final Answer:} Punjab.