D. The rental cost in dollars, for each paddleboard.
Explanation:
You were in the right track with linking 40 to the paddleboards.
But remember that t represent the total cost and not the total number of paddleboards and kayak, and also since p represent the number of paddleboards (while k represent the number of kayak)
To find t, you need to sum the cost of all the kayaks and the sum of all the paddleboards.
=> t = 25k + 40p
with 25k = total cost of all the kayaks
and
with 40p = total cost of all the paddleboards => in others words the total number of paddleboards (p) * the cost of each paddleboards (40)