Question

Which type of message will you use to send a private message to someone on Twitter?

Answer 1

The correct option is D.
Twitter is an online news and social networking website, where communication takes place in form of tweets. Tweets can only be 280 characters long. 
For those people who are using twitter, a direct message can be used to send a private message to someone about their tweet or other relevant issues. In order to be able to receive direct messages from anyone, one has to has to activate the option of receiving direct messages from anyone.

Answer 2

Which type of message will you use to send a private message to someone on Twitter?

I'd say D. a direct message.