________ car is parked outside?
Which of the following words CORRECTLY completes the sentence above?

Who’s is a contraction linking the words who is or who has, and whose is the possessive form of who.
This sentence is asking who owns or possesses the car that is outside, so we need to use the possessive form of the pronoun who, which is whose.
Who’s is incorrect in this sentence because Who’s car is parked outside would mean who is car is parked outside.
The other two answers are simply incorrect spelling.