The correct sentence is: "The bike on the bike rack is blue."
The other sentences contain grammatical errors. Here’s a brief explanation of the errors:
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"The sidewalks in the courtyards has been decorated with chalk." - The subject "sidewalks" is plural, so it should be "have" instead of "has."
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"The cars in the parking lot belongs to the teachers." - The subject "cars" is plural, so it should be "belong" instead of "belongs."
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"The surface of the parking lots were redone this summer." - The subject "surface" is singular, so it should be "was" instead of "were."