For each question you frame you can give the answer explanation too. From admin dashboard navigate to Mock Tests > [your test] > Quiz builder > Multiple Choice Question/Numerical Question > Question builder.
In question builder scroll down to the bottom. You will see Explanation section.
Once you have entered your explanation click on Save.
You can also load videos for your explanations. It would also show up in the preview.
To add explanation video, click on Load Video
In the next page, you have option to upload video from your computer or sync from dropbox or alternatively you can embed video from Youtube and Vimeo