The protocol and security level that libopenssl uses.
Best practice: Try your application with Tlsv1_2. That might not work.
Use Sslv23 for maximum compatibility. See SSL_CTX_new(3) for more
details.
SSLv2 was banned by RFC 6176 which contains a dire list of its
shortcomings.
Older versions of OpenSSL do not support Tlsv1_1 and Tlsv1_2. You will be able to link with such a version, but will get an error about an undefined symbol at runtime if you try using the unsupported version.