Option (D) $\mathbf{1000}$
$Q_{An} =Q_{C} \oplus Q_{D}, \ Q_{Bn} = Q_{A}, Q_{Cn}=Q_{B} \ \text{and} \ Q_{Dn}=Q_{C}$
$$\begin{array}{|c|c|c|c|} \hline \bf{Q_A} & \bf {Q_B}& \bf {Q_C} & \bf{Q_D }\\\hline1&1&1&1\\ 0&1&1&1\\0&0&1&1\\ 0&0&0&1 \\\hline \mathbf{1}&\mathbf{0} &\mathbf{0}&\mathbf{0}\\\hline\end{array}$$