Answer
(a) $F'(v)$ represents rate of change of fuel economy.
(b) See the image below.
(c) about $55mi/h$
Work Step by Step
(a) As we know, the graph of $F(v)$ is the fuel economy as a function of speed $v$ ($mi/h$).
So, the $F'(v)$ is going to be the rate of change of the fuel economy with respect to speed. It represents fuel economy change for any given $v$.
(b) See the image above
(c) The most convenient speed to save on gas is between $50$ - $55$ $mi/h$
