What mode is used when several computers are sharing files without a wireless access point?

Ad-hoc mode is used when several computers connect directly to each other for file sharing without utilizing a wireless access point or any central device. In this configuration, each computer acts as both a client and a server, enabling a peer-to-peer communication setup. This mode is particularly useful in scenarios where a quick and temporary network is needed, such as in a small group setting where users want to share files quickly without the complexity of setting up a traditional network infrastructure.

In contrast, infrastructure mode involves a central access point, which facilitates communication between devices in a more structured and managed environment, making it unsuitable for situations where there's no wireless access point. Bridge mode is used to connect two or more network segments, allowing them to function as a single network, and it typically involves more complex setups and configurations. Mesh mode is characterized by a network topology where each node can connect to multiple nodes around it, allowing for greater redundancy and coverage but also requiring a more complex infrastructure to interconnect all devices.

The choice of ad-hoc mode aligns perfectly with the need for direct, decentralized file sharing among multiple devices without reliance on additional infrastructure, making it the correct answer.