Final answer:
The cost of 5 plastic chairs is $75.
Step-by-step explanation:
To find the cost of 5 plastic chairs, we need to multiply the cost of one chair by the quantity of chairs. Here, the cost of one plastic chair is $15. So, the cost of 5 plastic chairs is:
Cost of 5 plastic chairs = Cost of 1 plastic chair × Quantity of chairs
Cost of 5 plastic chairs = $15 × 5 = $75
Therefore, the cost of 5 plastic chairs is $75, which corresponds to option a.