In which one of the following sentences is the apostrophe used correctly?
a. the train began it's journey across country.
b. i put the letter back into it's file.
c. the cat licked it's paws while the dog watched.
d. it's up to you to decide what to do.

D. It's up to you to decide what to do.
Possessive noun usually includes an apostrophe such as Nene's bear, plant's soil, earth's mass and a lot more. Therefore, the possessive noun in the new coat belonging to Sharon is Sharon's that is Sharon's new coat. Add 's to those singular or plural nouns that do not end in s however, add only an apostrophe to plural nouns that already ends in s. Moreover, possessive nouns can be personal pronouns too. Personal pronouns actually shows or displays ownership of something especially in a sentence plus it does not the 's form of the word. Possessive pronouns inlude we, mine, her, his, ours, theirs and a lot more.