Correct option is (D) Assertion (A) is false and Reason (R) is true.
While chat is usually a real-time conversation, email is generally not real-time. Emails are sent and received with delays, and there's no guarantee the recipient will see or respond immediately.
For chat, being online is generally necessary for real-time interaction. However, for email, you don't need to be online all the time. You can send and receive emails once you are online, but they aren't typically live interactions.