A company has just experienced a ransomware attack resulting from employees' credentials being used by a remote attacker after being compromised by technical means. The incident response team needs to provide recommendations in the incident report for improving the organization's cybersecurity posture. Which of the following recommendations would be the MOST effective?
a. Conducting security awareness training for employees
b. Upgrading the firewall software
c. Running regular vulnerability scans
d. Implementing multi-factor authentication