The order of arrangement of products according to their decreasing importance is:

Penicillin- Biogas – Citric acid – Curd

Penicillin is the most important product for the welfare of human society. It is an antibiotic, which is used for controlling various bacterial diseases. The second most important product is biogas. It is an eco-friendly source of energy. The next important product is citric acid, which is used as a food preservative. The least important product is curd, a food item obtained by the action of lactobacillus bacteria on milk

Hence, the products in the decreasing order of their importance are as follows: Penicillin- Biogas – Citric acid – Curd