The correct response is:
For each increase of 1 day, the cost increases by $25.
To determine this, you typically look at the slope of the graph representing the cost as a function of the number of days. If the cost increases consistently with each additional day at that rate, that would indicate that for every additional day rented, the cost goes up by a specific amount (in this case, $25). If you have the graph, you would look for how much the cost increases for that consistent unit increase in days.