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

Answer : D. It's up to you to decide what to do.

Based on the choices above, only the sentence in letter D is using apostrophe correctly. An apostrophe is a punctuation mark which refers to the possession of something such Trisha's bag or it also marks the omission of certain letters or numbers within a word. In the sentence in letter D, it can be seen that the apostrophe is used to omit the "i" in the word "is" joining it to "It" creating the word "It's". 

Can you insert a picture?