Colorado real estate contracts must be:

Option A is incorrect because verbal contracts for real estate transactions are not enforceable in Colorado. While some contracts can be verbal, real estate agreements fall under the Statute of Frauds exception requiring written documentation.

Option C is incorrect because while witnessing contracts is good practice, Colorado law does not require real estate contracts to be witnessed to be enforceable. The writing requirement is the key element, not witnessing.

Option D is incorrect because notarization is not required for real estate contracts to be enforceable in Colorado. While notarization may be required for certain documents like deeds, it is not a prerequisite for contract enforceability.