Answer : 4. "Mail Merge"
Explanation :

1. Mail Merge is a facility in MS Word using multiple or different letters, invitation cards, and personal letters that can be sent to many people in a minimum time.

2. This feature is more useful in a situation when a user has to deliver a letter or invitation card to thousands of people.

3. We can understand mail merge with an example. Suppose we have to send a letter or card to 1000 different people with their names and addresses, in that case, we will have to copy and paste that letter 1000 times for all those people, and then a lot of time will be wasted. A database will be created which will contain the names, addresses and other information of all the recipients. This database file is in MS Excel or MS Access.