The correct sentence is:
The bike on the bike rack is blue.
The other sentences contain subject-verb agreement errors. Here’s a brief correction for each:
- "The cars in the parking lot belong to the teachers." (Subject "cars" is plural, so the verb should be "belong.")
- "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.")