What item, about the size of a credit card, allows access to a network and its resources?

The correct choice is a smart card. A smart card is a physical device that resembles a credit card in size and shape, and it typically contains embedded integrated circuits that can process data. These circuits enable the card to securely store and manage various forms of sensitive information, such as authentication credentials, cryptographic keys, or personal identification data.

Smart cards are widely used for accessing secure networks and resources because they provide strong security features, including two-factor authentication. When used in conjunction with a reader, the smart card can grant users access based on the stored information and cryptographic functions, ensuring that only authorized individuals can connect to the network.

While security tokens and digital certificates also play roles in securing network access, they function differently: security tokens are often used in two-factor authentication scenarios but are generally key fobs or similar devices, and digital certificates are software-based and not physical items like smart cards. USB drives can store data and software but typically do not have the specific built-in security and authentication features that enable them to be used as access devices in the same manner as smart cards.