The word "ancient" refers to something very old or from a distant past. The antonym of "ancient" is modern, which refers to something from the present time or recent times.
\begin{itemize}
\item old: This is not the opposite of "ancient" since both are terms related to age, but "ancient" refers to something from a much older time.
\item ascent: This refers to the act of rising or going up, which is unrelated to the meaning of "ancient."
\item descent: This refers to going down, and while it's an antonym of ascent, it doesn't relate to "ancient."
\item modern: This is the correct antonym, as it refers to something belonging to the present time, the opposite of ancient.
\end{itemize}
\boxed{\text{modern}}