Let's match the size reduction equipment with their corresponding methods:
- P. Hammer mill operates by Impact (option 2), so \( P-2 \).
- Q. Burr mill operates by Attrition (option 4), so \( Q-4 \).
- R. Crushing rolls operate by Compression (option 1), so \( R-1 \).
- S. Rotary knife operates by Cutting (option 3), so \( S-3 \).
Thus, the correct answer is option (D).