Explanation:

- Option 1: under - Generally used to show something is beneath or below in rank or position. Doesn't fit well here.

- Option 2: in - Correct usage to express changes in price, cost, or quantity. "A decrease in the price"

- Option 3: of - Used to indicate possession or relation. It doesn't accurately express the decrease within the context.

- Option 4: to - Used to indicate direction or limit, such as a range or price drop to a specific number.

- Option 5: on - Indicates location or position, or when something is based on. Not suitable for this context.