Your network contains an Active Directory domain named contoso.com. The domain contains Windows 10
Enterprise client computers.Your company has a subscription to Microsoft Office 365. Each user has a mailbox that is stored in Office 365
and a user account in the contoso.com domain. Each mailbox has two email addresses.
You need to add a third email address for each user.
What should you do?

A.
From Active Directory Users and Computers, modify the E-mail attribute for each user.

B.
From Microsoft Azure Active Directory Module for Windows PowerShell, run the Set-Mailbox cmdlet.

C.
From Active Directory Domains and Trust, add a UPN suffix for each user.

D.
From the Office 365 portal, modify the Users settings of each user.

Explanation:
We can use the Set-Mailbox cmdlet to modify the settings of existing mailboxes.
The EmailAddresses parameter specifies all the email addresses (proxy addresses) for the recipient, including
the primary SMTP address. In on-premises Exchange organizations, the primary SMTP address and other
proxy addresses are typically set by email address policies. However, you can use this parameter to configure
other proxy addresses for the recipient.
To add or remove specify proxy addresses without affecting other existing values, use the following syntax:
@{Add=”[<Type>]:<emailaddress1>”,”[<Type>]:<emailaddress2>”…;
Remove=”[<Type>]:<emailaddress2>”,”[<Type>]:<emailaddress2>”…}.
Incorrect Answers:
A: You cannot use the E-mail attribute in Active Directory Users and Computers to add email addresses.
C: A UPN (User Principal Name) is used for authentication when you enter your credentials as
username@domainname.com instead of: domainname\\username. A UPN suffix is not an email address.
D: Users’ email addresses are not configured in the User settings in the Office 365 portal.

https://technet.microsoft.com/en-us/library/bb123981(v=exchg.160).aspx