Other / Mixed question from NEET 1990 (AIPMT)

Answer: (b)

1. A more positive grid voltage increases the plate current $I_p$.
2. Larger $I_p$ means a larger drop across the load, so the plate voltage $V_p = V_{pp} - I_p R_L$ falls.
3. Output therefore goes down when input goes up: phase difference $\pi$.
4. So the answer is (b) $\pi$.