The sun's declination is 0° at the a. summer and winter solstice b. summer and winter equinox c. vernal and autumnal solstice d. vernal and autumnal equinox

-- "Declination zero" means the object is in the sky at some point directly over the Earth's equator.  

-- If it's the sun and it appears to be over the equator, then that tells us that the Earth's axis is not tilted toward or away from it.  

-- That in turn tells us that the Earth is at one of the two equinoxes in its orbit, either the Spring one or the Autumn one. (D)

-- (The first days of Summer and Winter coincide with solstices, not equinoxes.)  

The sun's declination is 0 degrees at the vernal and autumnal equinox hopefully this helps!