Which of the following actions requires a real estate license to be legal?

The action that requires a real estate license to be legal among the options given is listing a property for sale. In Virginia, as in many states, individuals must hold an active real estate license to engage in the business of real estate transactions, which includes listing properties. This is because listing involves representing the seller and facilitating the sale, which requires knowledge of laws, contracts, and market conditions.

Negotiating closing costs for someone else, although it may seem related to real estate transactions, can fall under the realm of informal assistance between friends and does not constitute an official representation that would require licensure. Consulting on mortgage options does not require a real estate license either; this can be done by individuals as long as they are not acting as agents in the sale of properties. Conducting property showings requires a license if done as part of the brokerage service, but it is generally considered part of licensed real estate activities like listing and selling.

Hence, among the options, the act of listing a property for sale distinctly requires a real estate license to ensure compliance with legal standards and protect consumer interests.