Which of the Following Better Explains 'Competitive Bidding

Which of the following better explains 'Competitive Bidding'? 

A)  This is a situation where a supplier asks for orders from different buyers. 
B)  This is a situation where a buyer asks for bids from different suppliers. 
C)  This is a situation where a buyer asks for information from one supplier. 
D)  This is a situation where a buyer asks for bids from different suppliers, creating a level playing field.