What feature distinguishes smart cards from proximity cards?

The distinguishing feature of smart cards is the presence of embedded microprocessors. This allows smart cards to perform complex operations, such as data processing, authentication, and secure communication, which proximity cards cannot do since they are typically limited to simply transmitting data stored in their tags via radio frequency.

Smart cards can store encrypted data and support applications like digital signatures and multiple access permissions, making them more versatile for security purposes. The microprocessor enables smart cards to interact with terminals and process transactions dynamically, while proximity cards primarily rely on passive RFID technology to transmit a fixed ID number to a reader without any onboard processing capability.

In contrast, while other options may mention aspects of size or cost, they do not accurately represent the key technological advancement that sets smart cards apart from proximity cards in the realm of access control systems.