Step 1: Understanding the Question:
The Reynolds number ($R_e$) characterizes the nature of fluid flow:
\begin{itemize}
\item If $R_e<2000$, the flow is streamlined or laminar.
\item If $2000<R_e<3000$, the flow is unstable or transitional.
\item If $R_e>3000$, the flow is turbulent.
\end{itemize}
Step 2: Check Options:
We need to identify the value corresponding to streamlined flow ($R_e<2000$).
\begin{itemize}
\item 900 is less than 2000.
\item 2100 is in the transition region.
\item 2900 is in the transition region.
\item 4000 is turbulent.
\end{itemize}
Thus, 900 is the correct value for streamlined flow.