Answer: B.The cost of rental fee only
Explanation:
The $35 is the cost of the rental fee only. The person would then have to pay a further $15 for every hour they use the boat.
Essentially, the $35 is the fixed cost associated with the rental and the $15 is the variable cost to do so. If this were in a linear equation format it would take the form: y = 15x + 35.