Take a look at the graph of the function C(v): For which value v0 do you find a minimum?
The lowest emission that can be reached is marked by the horizontal blue line. It corresponds to C = 140 g/km. This value is reached at v0 = 75 km/h.
If you know how to find minima and maxima, you can compute the answer directly, by taking the derivative of our function:
2
C(v) = 0.045 v - 6.75 v + 393
The derivative is
C'(v) = 0.090 v - 6.75
For which value v0 is there an extremum? The equation to solve is
0 = 0.090 v0 - 6.75
and the solution is v0 = 75 km/h. Inserting this value in the equation for the function C(v), you find that C(v0) = 139.875. So the answer is 140 g/km.