Explanation
Isolating the infected attachment is the best course of action for the analyst to take to prevent further spread of the worm. A worm is a type of malware that can self-replicate and infect other devices without human interaction. By isolating the infected attachment, the analyst can prevent the worm from spreading to other devices or networks via email, file-sharing, or other means. Isolating the infected attachment can also help the analyst to analyze the worm and determine its source, behavior, and impact. References:
https://www.security.org/antivirus/computer-worm/
https://sec.cloudapps.cisco.com/security/center/resources/worm_mitigation_whitepaper.html