In requesting an SSL cert, my issuer gives me a 4 or 5 choices for selecting the type of apache running on my server. Examples include apache openssl and apache 2.
If all I have done is install unmodified Virtualmin on top of Centos 6.3, how would I find out the correct answer?
Actually, the webserver software in use should not have any influence on the actual certificate at all. 
Certs are not created for any specific web server.
My assumption is that this choice is merely there to present you with installation instructions, which possibly do differ between applications, during the process. So going with “Apache 2” should be fine.