Final answer:
The constant of proportionality is the walking speed of 2 km/h. To walk 2 km it takes 1 hour, to walk 4.5 km it takes 2.25 hours, and to walk 5 km it takes 2.5 hours according to Option b which is correct.
Step-by-step explanation:
The constant of proportionality in this context is the walking speed. If distance is proportional to the time, the ratio of the distance traveled to the time it takes is constant; this is the average speed. Here we're looking for the time it takes to walk certain distances at a given speed.
Option b is the correct one: 2 km/h. To solve for the time, divide the distance by speed. For 2 kilometers, the calculation is 2 km ÷ 2 km/h = 1 hour. For 4.5 kilometers, it's 4.5 km ÷ 2 km/h = 2.25 hours. For 5 kilometers, 5 km ÷ 2 km/h = 2.5 hours.
Remember that the constant of proportionality, in this case, is a direct representation of speed, and the units for time when distance is given in kilometers and speed in km/h will be hours.