How to force a newline unless this increases the total number of lines

Question

Is it possible to force a newline in a specific position of a paragraph but only if this does not increase the total number of lines?

I tried by using \penalty with different values, and found that -10000 always forces the newline while -9999 does not enable the newline even if the total number of line breaks is the same.

Here is a possible test document:

\documentclass{article}
\newcommand\X[1]{
  \penalty-9999 % please break here if you need to break somewhere                     
  \textbf{(#1)}
}
\begin{document}
\begin{minipage}{5cm}
  Which number?
  \X{A} 5
  \X{B} 7
  \X{C} 4
  \X{D} 3
\end{minipage}
\end{document}

Answer 1

\documentclass{article}
\newcommand\X[1]{% keep egreg happy
  \unskip % remove any space before this command, so we add our own
  \hskip 0pt plus .2\hsize % end line up to 20% short
  \penalty-50 % please break here if you need to break somewhere  
  \hskip 0pt plus -.2\hsize\relax % cancel skip if don't break
  \space  % and do normal space                
  \textbf{(#1)}% as above
}
\begin{document}
\begin{minipage}{5cm}
  Which number?
  \X{A} 5
  \X{B} 7
  \X{C} 4
  \X{D} 3
\end{minipage}
\end{document}