Answer: Change the account's password
Step-by-step explanation:
Since Martha wants to ensure that no one uses the colleague's user account until he returns, the best thing for her to do is to change the account's password.
If she changes the accounts password, then she can be the one that will have access to the account and then ensure its safety. It should be noted that the colleague's account can contain vital data and information, therefore it's vital to keep it safe.