Which of the following prevents malicious programs from attacking a system?

An anti-virus program actively scans files and running processes for known malware signatures and heuristic behavioral patterns, intercepting and neutralizing malicious programs before they can execute or spread. It provides real-time protection at the endpoint layer, which is the primary defense specifically targeting malicious code. This makes it the correct answer among tools that serve authentication or network-level purposes.

Smart cards are hardware-based authentication tokens used for identity verification and access control, and provide no capability to detect or block malicious programs.

Biometric devices authenticate users based on physical characteristics and have no mechanism to scan for or prevent the execution of malicious software.

A firewall filters network traffic based on rules but does not inspect files on the local system for malware, and cannot stop malicious programs already present or delivered through permitted connections.