Rule of economic reasoning: "Draw the graph!"
Choice "A" is correct. When demand increases and supply has not increased (as implied by the question), suppliers will raise the price of the product and more product will be bought (but the supply curve does not change). Because consumers are demanding more product than is available, they are "willing" to pay a higher price.

Choice "d" is incorrect. Although buyers would pay higher prices and purchase more products, the supply "curve" has not changed. Therefore, the quantity supplied remains the same.
Choice "b" is incorrect. Because consumer demand has increased (not decreased).
Choice "c" is incorrect. An increase in demand has an indeterminate (and irrelevant) impact on the number of buyers. For example, there could be the same number of buyers in the market, but that each demands a higher quality.