$d = 5 \sin \left( \dfrac{2\pi}{13} \right)t + 9$
When t = 13/4
$d = 5 \sin \left[ \left( \dfrac{2\pi}{13} \right) \cdot \dfrac{13}{4} \right] + 9$
$d = 5 \sin \left( \dfrac{\pi}{2} \right) + 9$
$d = 14 ~ \text{m}$
When t = 39/4
$d = 5 \sin \left[ \left( \dfrac{2\pi}{13} \right) \cdot \dfrac{39}{4} \right] + 9$
$d = 5 \sin \left( \dfrac{3\pi}{2} \right) + 9$
$d = 4 ~ \text{m}$
The depth of the high tide is 14 meters and the depth of the low tide is 4 meters.
Answer = [ C ]
