Answer:
Caustic soda would be least affected by a gas curtailment
Explanation:
Let, 
X1= ammonia
X2= ammonium phosphate
X3= ammonium nitrate
X4 = Urea
X5= hydro-fluoric acid
X6= Chlorine
X7= Caustic soda
X8= Vinyl chloride monomer.
Agri chem’s current natural gas usage 
= (1200 × 8 + 540 × 10 + 490 × 12 + …)                  
= 85,680,000 cu. ft. per day  
When, the curtailment is 20%, availability is 
= 0.8 X  85,680 
= 68,554,000 cu. ft. per day
Therefore, the gas constant
 = 8X1 + 10X2 + 12X3 + 12X4 + 7X5 + 18X6 + 20X7 + 14X8 ≤68,544
When, the curtailment is 40% availability is 
= 0.6 X 85,680 
= 51,408,000 cu. ft. per day
Constant = 8X1 + 10X2 + 12X3 + 12X4 + 7X5 + 18X6 + 20X7 + 14X8≤ 51,408