In which of the following circumstances must you provide your California contractor's license number?

Providing a California contractor's license number is essential in multiple contexts to ensure compliance with state regulations and maintain transparency in business practices. Each of the circumstances mentioned requires the inclusion of your contractor's license number, which serves different purposes.

On contracts, the license number helps to validate that the contractor is legally established and authorized to perform the specified work. This protects clients by ensuring they are hiring a legitimate professional who meets the state’s licensing requirements.

In advertising, including your license number promotes trust and credibility. It informs potential clients that you are properly licensed and insured, which can influence their decision-making when choosing a contractor.

"Preliminary 20-day Notices" are legal documents sent to property owners and other relevant parties to inform them of a contractor's involvement in a construction project. Including the license number in these notices is vital as it helps to identify the contractor and ensures compliance with legal requirements under California's mechanics lien laws.

Thus, the requirement to provide your contractor's license number spans across contracts, advertising, and legal documents like Preliminary 20-day Notices, making it necessary to include this information in all these circumstances.