"The moon is aligned with Earth and the sun when full or new, so its gravity combines with the sun's gravity to create an even stronger gravitational pull."

Which option has the best word choice for this sentence?

︎ The moon is aligned with Earth and the sun when full or new, so his gravity combines
with the sun's gravity to create an even stronger gravitational pull.

︎ No change

︎ The moon is aligned with Earth and the sun when full or new, so it's gravity combines
with the sun's gravity to create an even stronger gravitational pull.

︎ The moon is aligned with Earth and the sun when full or new, so their gravity
combines with the sun's gravity to create an even stronger gravitational pull.