$\left|\begin{array}{ll}
\mathrm{a} & \mathrm{d} \
\mathrm{b} & \mathrm{c}
\end{array}\right|=\mathrm{ad}-\mathrm{bc} \Rightarrow \mathrm{ad}=\mathrm{bc}$
Case-I Exactly 1 no. is used
⇒ All singular =4C1
Case-II Exactly 2 no. is used
⇒4C2×2×2
Case-III Exactly 3 no. is used
None will be singular
Case-IV Exactly 4 No. is used
ad=bc
⇒2×9=3×69−−2⇒4C1×21
Total =36