$$\overset{\text{Process}}{\begin{array}{|c|c|c|}\hline\\
\textbf{process}& \textbf{Arrival Time}& \textbf{Burst time} \\\hline
P_1& \text{0}& 10 \\ \hline
P_2& \text{2}& 20 \\ \hline
P_3& \text{6}& 30 \\\hline
\end{array}}$$
Shortest remaining time first :- Select the process with the smallest amount of remaining time among all the process to execute first.
So here we will $1^{st}$ select $P_1$ then $P_2$ and at last $P_3$.
It is mentioned in question
Do not count the context switches at time zero and at the end
So, we will have take only $2$ context switch into account i.e. $P_1$ to $P_2$ and $P_2$ to $P_3$.
$\therefore$ Option $B.$ $2$ is the correct answer.