My answer is B. log in during non-working hours and C. email forwarding to an external domain
UEBA (User and Entity Behavior Analytics) is a security technique that uses machine learning algorithms to identify abnormal behavior within an organization's network. In this scenario, two behaviors that likely triggered an UEBA alert are the employee logging in during non-working hours and forwarding an increased amount of emails to an external mail domain. These behaviors deviate from the employee's normal patterns of activity and may indicate an attempt to exfiltrate sensitive information to a competitor.
A. domain belongs to a competitor is not a behavior, it's an information that's used in the investigation process.
D. Log in from a first-seen country and E. increased number of sent mails also can be important indicators, but they are not enough to trigger UEBA alerts.