What is the determinant of bmatrix 5 & 3 \\ 2 & 4 bmatrix?

For a $2\times 2$ matrix $\begin{bmatrix} a & b \\ c & d \end{bmatrix}$, the determinant is $ad-bc$. Here $a=5$, $b=3$, $c=2$, and $d=4$, so the determinant is $(5)(4)-(3)(2)=20-6=14$, choice D. Choice B, $20$, is only the product $ad$ with the subtraction of $bc$ left out. Choice A, $6$, is only $bc$. Choice C, $26$, adds the two products ($20+6$) instead of subtracting. Remember that the determinant is the main-diagonal product minus the other-diagonal product, in that order.