The correct Answer is option D) $2.50
Let me explain how:
Differentiate y with respect to x
dy/dx= d/dx(3.75 + 2.50(x - 3))
dy/dx= 0+2.5(1) - 0=2.5
Hence, from the above calculations it is proven that the rate of change of the cost in dollars
=dy/dx
=2.5