Which of the following mechanisms ensures that a user cannot later deny having requested a product?

The correct choice highlights the use of a receipt or token as a mechanism to ensure that a user cannot later deny having requested a product. This is grounded in the concept of non-repudiation, which is essentially the assurance that someone cannot deny the validity of something. A receipt or token serves as proof of a transaction or interaction, effectively providing evidence that the user participated in the action of requesting the product.

When a user receives a receipt or token, it is typically issued after their request has been processed and carries details about the transaction, including timestamps, item descriptions, and unique identifiers. This documentation serves as a record that can be referenced later in any disputes regarding the request.

In contrast, while a digital signature can also provide non-repudiation by confirming the authenticity of a user's action, the presence of a receipt is more directly tied to the action of requesting a product itself. Meanwhile, public key infrastructure primarily facilitates secure communications and may include elements of verification but does not specifically address user-deniability in the context of requests. On the other hand, encryption algorithms are concerned with protecting data confidentiality rather than proving actions taken by users.