Given: $\textsf{(BCA9)}_{16}$
$1011\quad 1100\quad 1010\quad 1001$
For octal number system: grouping of three- three bits from right to left
$\begin{array}{c c c c c c } 1 & 011 & 110 & 010 & 101 & 001 \\ 1 & 3 & 6 & 2 & 5 & 1 \end{array}$
Answer: option D) $(1 3 6 2 5 1)_{8}$