A: Hi,
You can use any you want; the main purpose of this plugin isn't to provide you with a certificate but to let you manage it and keep track of the errors, mixed content, etc. The plugin allows you to scan your content, get reports & alerts, etc - please scroll up to read more about all the features or check https://wpforcessl.com/ :)

You can install a free Let's Encrypt certificate via our plugin though :)