C. 75%
Step-by-step explanation:
List Price= Net Price ÷ Complement of trade discount rate
120 = (120 ×25 ÷100 = 90) ÷ x
120 = 90 ÷ x
x = 90÷120 ×100 = 75%

Compliment = 90÷120×100 = 75%

Hence
Trade discount is 25% so the complement is 75% (100% -- 25%)