The correct answer is - d. spring.
The unusually high tide is called spring tide.
This tide is higher than the average tide, and much higher than the tide in the period when it is the lowest.
The spring tide always appears after a new moon or a full moon, which is a clear indicator what influences it. The higher a tide is, the bigger the influence on the water bodies by the Moon's gravitational pull, and vice versa.This tide is actually the biggest difference between the high tide and the low tide.