The correct answer is C.
The demand function represents the quantity of a certain good or service that consumers are willing to purchase in the market at different price levels. The law of demand states that there is an inverse relationship between price and quantity demanded (ceteris paribus, hence, given that the rest remains equal).
If the amount demanded has increased for all price levels, the demand curve has experienced a shift to the right. If the amount demanded has decreased for all price levels then the shift has been to the right.
These shifts can be triggered due to several factors such as variations in consumer income, changes in consumer preferences or variations of prices of related goods.