Let x represent the number of days and let y represent the amount charged for x days.
We were told that she charges $6.25 for 5 days and $8.75 for 7 days. In either cases, the amount that she charges per day is
6.25/5 = 8.75/5 = 1.25
Thus, the equation representing this relationship is
y = 1.25x
where 1.25 represents the slope of the equation
Option C is correct