Final answer:
The net change in the stock was a $3 decrease, calculated by subtracting the final value ($21) from the initial value ($24).
Step-by-step explanation:
To determine the net change in the stock from Monday ($24 a share) to Friday ($21 a share), subtract the value of the stock on Friday from the value on Monday:
Net change = Friday's stock value - Monday's stock value
Net change = $21 - $24
Net change = $3 decrease
Therefore, the net change in the stock from Monday to Friday is a $3 decrease, which corresponds to option A. $3 decrease.