Answer: Third option.
Step-by-step explanation:
The equation of the line in Slope-Intercept form is:

Where "m" is the slope of the line and "b" is the y-intercept.
In this case we have the following equation that models the relation between the cost in dollars (C) per day and the number of miles (m) Janelle drives in one day:

You can identify that that the slope is
and the y-intercept is
.
This indicates that for every increase of
in the input variable "m", the value of the output variable "C" will increase by 
Therefore, the slope,
, means that the cost for renting the car on a particular day increases by
of a dollar for each mile she drives on that day.