The correct sentence from the given sentences is 'The bike on the bike rack is blue.' In the other three sentences, the error is in subject-verb-agreement. Subject-verb-agreement exemplifies that the subject and the verb in a sentence should be the same in numbers and tense. This means that with the singular noun, the singular verb will be used and with the plural noun, the plural verb will be used. In the first sentence, the subject 'cars' is plural, but the verb 'belongs' is singular. In the second sentence, the subject 'sidewalks' is plural, but the verb 'has' is singular. In the third sentence, the subject 'surface' is singular, but the verb 'were' is plural.
