Question

1. during the vernal equinox, what occurrence takes place that happens only twice a year?

the suns rays are perpendicular to the equator, causing 12 hours of daylight on both northern and southern hemispheres.
the suns rays are perpendicular to the prime meridian, causing 12 hours of daylight on both northern and southern hemispheres.
the suns rays are perpendicular to the tropic of capricorn, causing 12 hours of daylight on both northern and southern hemispheres.
the suns rays are perpendicular to the tropic of cancer, causing 12 hours of daylight on both northern and southern hemispheres.

Explanation:

During the vernal equinox (and autumnal equinox), the Sun's rays are perpendicular to the Equator. This results in approximately 12 hours of daylight across the globe (both hemispheres). The Prime Meridian is a longitude line and not relevant to solar - daylight distribution. The Tropic of Capricorn is relevant during the December solstice (Southern Hemisphere summer), and the Tropic of Cancer is relevant during the June solstice (Northern Hemisphere summer).

Answer:

The Sun’s rays are perpendicular to the Equator, causing 12 hours of daylight on both Northern and Southern Hemispheres.