Can someone tell me what is the answer to this question is? I am assuming the answer is A. Please explain why it is correct or why it is not the correct answer?

The quantity supplied of a good is

Select one:

a. the amount that the producers are planning to sell at a particular price during a given time period

b. the amount the firm would sell if it faced no resource constraints

c. the same thing as the quantity demanded at each price

d. equal to the difference between the quantity available and the quantity desired by all consumers and producers