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