d
(d) Fundamental frequency of open organ pipe $ = \frac{v}{{2l}}$
Frequency of third harmonic of closed pipe $ = \frac{{3V}}{{4l}}$
$\therefore \frac{{3v}}{{4l}} = 100 + \frac{v}{{2l}}$
$ \Rightarrow \frac{{3v}}{{4l}} - \frac{{2v}}{{4l}} = \frac{v}{{4l}} = 100$
$ \Rightarrow \frac{V}{{2l}} = 200\,Hz.$