A: If the contact card shows Unsubscribed from email marketing, a user with Administrator or Marketing access can resubscribe the contact: click Add on the contact card, click Resubscribe for the email campaigns, then confirm with Resubscribe. Read more in How to resubscribe contacts.
The contact can also do it themselves, which is the only way if they opted out of specific email types. The contact can click "Unsubscribe" in your last mail campaign and is able to choose what they want to receive and not. If that is not possible, you can create a template with the tag {{SUBSCRIPTIONS}} and send to them. Then the contact will be able to handle their mail campaign subscriptions there. The page lists your active email types, see How to work with unsubscribe lists & email types.
Then you can use this template to send directly to the contact via a single mail and our mail apps.