The correct option is B Moderators cannot be available online 24 hours a day.
Online focus groups can be conducted either synchronously or asynchronously. In the former case, the researcher can use conferencing software to help participants converse in real time, although this can be problematic if some participants do not have access to the software. (A possible modern remedy for this might be instant messaging services.) Asynchronous focus groups can be conducted via an email distribution list, allowing people more time to compose their responses. However, a problem with this is that the moderator cannot be available online whenever someone is writing a message, and so it can be harder to keep the discussion on track and check the content of messages before they are posted.