The correct sentence is:
The bike on the bike rack is blue.
The other sentences contain grammatical errors related to subject-verb agreement. Here are the corrections for the other sentences:
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The surface of the parking lots was redone this summer. (The subject "surface" is singular, so it should take "was.")
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The sidewalks in the courtyards have been decorated with chalk. (The subject "sidewalks" is plural, so it should take "have.")
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The cars in the parking lot belong to the teachers. (The subject "cars" is plural, so it should take "belong.")