Final answer:
Default-contingent signals are appropriate to use when the quality of a product is difficult to convey after purchase. This includes situations where a money-back guarantee is offered.
Step-by-step explanation:
Default-contingent signals are appropriate to use when the quality of a product is difficult to convey after purchase. This means that option c, 'After purchase, quality is difficult to convey,' is the appropriate time to use default-contingent signals.
Default-contingent signals are signals that are automatically triggered based on a pre-agreed condition. For example, a money-back guarantee is a default-contingent signal that reassures buyers that if they are not satisfied with the quality of the product, they can get a refund.