Day after day, the earth made _ year-long journey around the sun

A. his

B. it's

C. its'

D. its

B or d i think is the right answer

God Bless

Day after day, the earth made its year-long journey around the sun.

His would be if you were referring to a male (animal, human, etc).

It's shows possession of something, the earth doesn't possess the year-long journey.

Its' is another way of showing possession if the thing that posses it already has an s on the end. (Example: Charles' car.)