\RequirePackage{xkeyval}
\define@boolkey{family}{a4paper}{}
\def\@hello{Missing `hello' parameter. }
\DeclareOptionX<family>{hello}{\edef\@hello{Defined `hello' to #1. }}
\def\@world{Missing `world' parameter. }
\DeclareOptionX<family>{world}[default]{\edef\@world{Defined `world' to #1. }}
\ProcessOptionsX<family>
\providecommand{\scenario}{%
  \csname ifKV@family@a4paper\endcsname
    Should not have read a4paper option. 
  \else
    Did not read a4paper option as expected. 
  \fi
  \@hello%
  \@world%
}