We need to match the product classes with their representative products based on their uses or sources.
Step 1: Biofuel
Biofuels are typically produced through fermentation or other microbial processes, and butanol is a common biofuel produced by fermentation.
Thus, \( P \) matches with 3.
Step 2: Bioplastic
Bioplastics are made from renewable biomass sources. Poly-lactic acid (PLA) is a common biodegradable bioplastic.
Thus, \( Q \) matches with 4.
Step 3: Industrial enzyme
Industrial enzymes like cellulase are used in various industrial processes, such as in the breakdown of plant biomass.
Thus, \( R \) matches with 1.
Step 4: Antibiotic
Cephalosporin is a well-known antibiotic used to treat a variety of bacterial infections.
Thus, \( S \) matches with 2.
Final Answer:
The correct answer is \( \boxed{D} \).