$\textrm{This is an alternating subtraction series with repetition. There are two different patterns here.}$
. $\textrm{In the first, a number repeats itself; then 3 is added to that number to arrive at the next number, which also repeats.}$
$\textrm{This gives the series 17, 17, 20, 20, 23,23 and so on.}$
$\textrm{Every third number follows a second pattern, in which 3 is subtracted from each number to arrive at the next: 34, 31, 28}$.
$\textrm{First Series: $17+3=20$,$20+3=23$}$
$\textrm{Second series: $34-3=31$,$31-3=28$}$
$\textrm{Option D}$