The correct answer is: C. Solar tidal force and lunar tidal force both coincide.
Spring tides are the highest high tides and the lowest low tides. They occur when the Sun and Moon are in alignment with Earth, either on the same side (new moon) or on opposite sides (full moon). This alignment causes the gravitational forces of the Sun and Moon to combine, creating stronger tides.
Option A is incorrect because the Sun and Moon are not always in line with Earth. In fact, they are only in alignment during new moons and full moons.
Option B is incorrect because the solar tidal force does not act opposite to the lunar tidal force. The two forces act in the same direction, but they have different strengths.
Option D is incorrect because spring tides are caused by the combined gravitational forces of the Sun and Moon.