​One reason demand curves slope downwards is

A) ​Marginal value increases with each purchase
B) Marginal value declines with each purchase
C) Total value declines with each purchase
D) ​All of the above

The correct answer is B) Marginal value declines with each purchase. As consumers buy more units of a good, the satisfaction they derive from each additional unit decreases. This means they are willing to pay less for each additional unit, so the demand curve slopes downwards. A) is incorrect because marginal value does not always increase with each purchase, and C) is incorrect because total value may not necessarily decline with each purchase.