Contractor license law does NOT require contractors to display their license numbers on which of the following?

The correct choice is related to the regulations governing contractor licensing and the specific requirements for displaying license numbers. Unlike flyers, building permits, and contracts—which are formal documents often used in the course of business where verification of licensure may be essential—purchase orders typically serve as internal documents for the buying and selling of goods and services, and are less formal compared to the other options listed.

Display of the contractor's license number is crucial on flyers because they are publicly distributed and serve to inform potential customers about the legitimacy of the contractor's business. Similarly, building permits and contracts are legal documents that directly impact the construction process, ensuring that all parties understand the contractor's credentials and compliance with state laws.

Conversely, purchase orders are primarily transactional documents used to request or confirm purchases and do not inherently require the same level of regulatory documentation or public accountability. Therefore, license numbers are not mandated on purchase orders, making this option the correct answer.