Which of the following requires a real estate license?

The requirement for a real estate license is particularly applicable in the case of selling time-share units. Time-share sales involve multiple ownership interests in a single property and typically require specialized knowledge of the legal structures surrounding time-sharing. Additionally, time-share transactions are regulated by specific laws and regulations that further necessitate the involvement of licensed real estate professionals to protect consumers' interests.

The other scenarios can often fall outside the scope of requiring a real estate license. For instance, an employee of a developer selling lots in a subdivision might be acting under the company’s real estate license, thereby not requiring a separate license. Renting out a condominium for a short period, like three weeks, often does not necessitate a real estate license unless the individual is holding themselves out as a real estate professional. Lastly, while selling more than 20 personally owned properties could imply a business activity, it doesn't automatically demand a real estate license unless that activity represents a pattern of behavior typical of a licensed broker. Thus, the sale of time-share units clearly establishes a licensing requirement due to its complexity and regulatory context.