Answer:
a. When the equipment is sold.
Step-by-step explanation:
As we know that
When someone sells or purchase a product, the services are attached to the product which is passed from the buyer to the seller that can be in terms of warranty i.e after-sales services, etc
So according to the given situation, the estimation of the warranty cost is $25 per time sold so the warranty cost should be recognized when the equipment is sold as it is attached to the product