You need to prevent this employee from sending emails to anyone at the firm

You are the help desk technician for a local real estate firm that uses Microsoft Exchange Online.
A former employee is sending unwanted emails to employees within the firm.
You need to prevent this employee from sending emails to anyone at the firm.
What should you do?

A.
Create a malware filter policy.

B.
Configure a message trace.

C.
Create a content filter policy.

D.
Create a transport rule.

Explanation:
You can use Exchange transport rules to look for specific conditions in messages that pass through
your organization and take action on them. Transport rules are similar to the Inbox rules that are
available in many email clients. The main difference between transport rules and rules you would set
up in a client application such as Outlook is that transport rules take action on messages while
they’re in transit as opposed to after the message is delivered. Transport rules also contain a richer
set of conditions, exceptions, and actions, which provides you with the flexibility to implement many
types of messaging policies.
Transport rules
https://technet.microsoft.com/en-us/library/jj919238%28v=exchg.150%29.aspx