The time of greatest variation between high and low tides occurring when Earth, moon, sun form a straight line. Spring tides alternate with neap tides throughout the year, occurring at two-week intervals.

• A. Mixed Tide
• B. Semidiurnal Tide
• C. Slack Water
• D. Spring Tide

Answer: (D)

Tides are strongest when the Sun, Moon, and Earth align and their gravitational forces combine. When they line up in a straight line, the pull on the oceans is reinforced, producing higher high tides and lower low tides—the largest tidal range. This is what a spring tide is, and it occurs approximately every two weeks during the new and full Moon phases.

In contrast, slack water is the calm, momentary pause when there’s no horizontal current around high or low tide. Semidiurnal tides produce two nearly equal high tides and two low tides each day, while mixed tides have two high tides of different heights. So the situation described—greatest variation in water level due to alignment of Sun and Moon—matches spring tides.