I doubt its true or i doubt it's true? Which one is grammatically correct?

The correct one is I doubt it's true.

The answer to this is I doubt it's true

This is because (i doubt its true) this sentence misses the apostrophe in the sentence meaning it is not a contraction therefore not making sense

The second sentence however it is contrast to the first information and it is yet very grammatical sentence. it has a contraction meaning it stands for it is and therefore making the sentence grammatically correct