Thank you for reaching out @RashmiR14. When you selecting Virtual Box platform for your QuickStart VM download there is a pop up box which may be blocked by your browser settings. Temporarily turn off your pop up blocker and you should be able to see the form and complete the download.
If not, let me know.
I have tested the 5.8 Quickstart VM download page and am able to select Virtual Box for the platform. At that point the pop up is triggered. If you are having an issue still you may need to clear your browser cache or try another browser.