Answer: Option D 
 
Step-by-step explanation: In simple words, the demand curve can be defined as the graphical representation of the price and the quantity demanded of a commodity. It depicts the demand function which states that when price of a good rises the demand of that goods decreases. 
In such a diagram the price is shown in the Y axis while the demand is shown in the X axis. 
Thus, from the above we can conclude that the correct option is D .