Evaluate the determinant $\begin{vmatrix} 1 & 2 & 3 \\ 0 & 4 & 5 \\ 0 & 0 & 6 \end{vmatrix}$.

The given matrix is upper triangular. The determinant of a triangular matrix is the product of its diagonal entries. Therefore, det = 1 × 4 × 6 = 24. Option B doubles the result (a common error), option C is the product of all non-zero entries, and option D takes only the last diagonal entry.