The correct answer is D) Moon being at its maximum declination. Tropic tides are caused by the Moon being at its maximum declination, which occurs when the Moon reaches its farthest northerly or southerly position relative to the celestial equator. This results in the greatest diurnal (daily) inequality, with the high and low tides being more extreme. The other options are incorrect because: A) the Moon crossing the equator does not directly cause tropic tides, B) both the Sun and Moon being near 0° declination would result in equatorial tides, not tropic tides, and C) the Moon being at perigee (closest to the Earth) affects the range of the tide, but not the diurnal inequality that defines tropic tides.