Explanation:

- Option 1, as it is: This option keeps the original phrase unchanged. The phrasing "it’s door" is incorrect due to the misuse of the contraction "it's", which means "it is".

- Option 2, as its: This option correctly uses "its" as the possessive form of "it", indicating that the door belongs to the car. "Its" is the correct word to use in this context.

- Option 3, No substitution required: This suggests the original phrase is correct as is, which is not true due to the grammatical error involving "it’s" instead of "its".

- Option 4, because it’s: This option is irrelevant since it suggests a different conjunction, "because", without addressing the possessive form needed for "its".

The correct answer is Option 2, as its.