Final answer:
The supply curve is positively sloping due to a direct relationship between price and quantity supplied; as price increases, so does the quantity producers are willing to supply, which is reflected in the upward slope of the supply curve.
Step-by-step explanation:
The supply curve is positively sloping because of a direct relationship between price and quantity supplied. This means that as the price of a good or service increases, the quantity that producers are willing to supply also increases. Conversely, when the price decreases, the quantity supplied decreases as well. This relationship is illustrated by the upward slope of the supply curve on a graph where price is on the vertical axis and quantity is on the horizontal axis. The concept of ceteris paribus, which means 'other things being equal', indicates that this relationship holds true when all other economic factors are constant.
The correct answer to why a supply curve is positively sloping is Option D: Direct relationship between price and quantity supplied.