d
The equation of motion is given as $y=5 \sin \frac{2 \pi t}{6}$
Here, $y=2.5 cm$
$\therefore 2.5=5 \sin \frac{2 \pi t}{6}$
$\Rightarrow \frac{\pi}{6}=\frac{2 \pi t}{6} \Rightarrow t=\frac{1}{2} s$
$\therefore$ The phase $=\frac{2 \pi t}{6}=\frac{2 \pi}{6} \times \frac{1}{2}=\frac{\pi}{6}$