We begin inside directory $\texttt{D3}$.
The parent of $\texttt{D3}$ is $\texttt{D1}$.
Therefore:
$\texttt{..}$
takes us from:
$\texttt{D3} \rightarrow \texttt{D1}$
From $\texttt{D1}$, we then follow:
$\texttt{D2}\rightarrow \texttt{D4}\rightarrow \texttt{a}$
So the complete relative pathname is:
$\texttt{../D2/D4/a}$
Hence:
$\boxed{\texttt{../D2/D4/a}}$