What does the prohibition of ex post facto laws prevent the government from doing?

Responses

trying and convicting a person when she is not present
trying and convicting a person when she is not present

passing a law today and punishing someone for her actions prior to its passage
passing a law today and punishing someone for her actions prior to its passage

restricting the rights of handgun ownership
restricting the rights of handgun ownership

holding a person without trial or explanation

holding a person without trial or explanation