d
If the fundamental frequency is $120 Hz$, which is also known as finst harmonic, then $2nd$ harmonic will be $=(120 \times 2)=240 Hz$
$3rd$ harmonic will be $=(120 \times 3)=360 Hz$
$4th$ harmonic will be $=(120 \times 4)=480 Hz$ and $30$ on.
So, all but $450 Hz$ will not be a harmonic, so it will be least harmonius with $240 Hz$. (option $D)$