FlexiContact Plus has better ways of preventing spam than using a captcha. But if you insist, there are several to choose from. This example has Google "Invisible" captcha enabled. Most of the time you won't have to do anything with the captcha to send the form, but it is doing its job silently in the background and will challenge the user if necessary. You can hover the mouse over the icon, bottom right, to see that it is loaded.
Try the other languages to see the same form in different styles with different captchas.