Đáp án:
$\begin{array}{l}
A = \left\{ {1;2;3;4;6;12} \right\}\\
D = \left\{ 2 \right\}\\
Khi:D \subset X \subset A\\
\Leftrightarrow X = \left\{ 2 \right\};X = \left\{ {1;2} \right\};X = \left\{ {2;3} \right\};X = \left\{ {2;4} \right\}\\
X = \left\{ {2;6} \right\};X = \left\{ {2;12} \right\};X = \left\{ {1;2;3} \right\};X = \left\{ {1;2;4} \right\}\\
X = \left\{ {1;2;6} \right\};X = \left\{ {1;2;12} \right\}\\
X = \left\{ {2;3;4} \right\};X = \left\{ {2;3;6} \right\};X = \left\{ {2;3;12} \right\}\\
X = \left\{ {2;4;6} \right\};X = \left\{ {2;4;12} \right\}\\
X = \left\{ {1;2;3;4} \right\};X = \left\{ {1;2;3;6} \right\};X = \left\{ {1;2;3;12} \right\}\\
X = \left\{ {2;3;4;6} \right\};X = \left\{ {2;3;4;12} \right\}X = \left\{ {2;4;6;12} \right\}\\
X = \left\{ {1;2;3;4;6} \right\};X = \left\{ {1;2;3;4;12} \right\}\\
X = \left\{ {2;3;4;6;12} \right\};X = \left\{ {1;2;4;6;12} \right\}\\
X = \left\{ {1;2;3;6;12} \right\}\\
X = \left\{ {1;2;3;4;6;12} \right\}
\end{array}$
