Explanation
The SIEM log shows that the user opened an email attachment named "Invoice.xlsx" and then executed a PowerShell script that downloaded and ran a malicious file from a remote server. This indicates that the attacker was able to bypass the application approve list by emailing a spreadsheet attachment with an embedded PowerShell in the file. This is a common technique used by malware authors to evade detection and deliver their payloads1.

Password history policies determines the number of unique new passwords that must associated with a user's account before an old password be reused. Essentially forcing users to create new passwords on a regular basis.
For this situation, forcing users to use new unique passwords would somewhat mitigate the issue.

NIST 800-53 provides a catalog of security and privacy controls related to the United States federal information systems. Reference: CompTIA Security+ Study Guide, Exam SY0-601, 4th Edition, Chapter 3: Architecture and Design, pp. 123-125

Explanation
BCP is to empower an organization to keep crucial functions running during downtime. This, in turn, helps the organization respond quickly to an interruption, while creating resilient operational protocols.