What incentive does an IRC provide to the sender?

The incentive that an International Reply Coupon (IRC) provides to the sender is the prepaid response capability. An IRC allows the recipient of a letter sent internationally to redeem it for postage, ensuring that the sender does not have to worry about the cost of replying. This makes it easier for senders to communicate across borders, as the reply postage is prepaid, removing the burden of incurring an extra expense for the recipient in providing a response.

The other options do not accurately reflect the function of an IRC. For instance, rapid delivery does not necessarily pertain to the IRC, as the coupon is not a guarantee of speed; instead, it facilitates the process of communication by making it easier for recipients to respond. Additionally, free express shipping is not a feature associated with IRCs, and while loyalty points might be part of some customer rewards programs, they are not related to the functions of an IRC.