The correct answer is this one: "is the one he expects." That is the complete adjective clause in the sentence 'Our car is the one he expects to buy.' Adjective clause are clauses which comprises of words that has a subject and a verb that provide description.
The complete adjective clause is number 3, which is the one he expects. An adjective clause is used to describe a subject, may it be a noun or a pronoun. The clause the one he expects describes what type of car the subject wants to buy.
