In which scenario would a registrant be acting outside of REBBA guidelines?

Guaranteeing an increase in property value is a scenario that clearly contravenes the guidelines established by the Real Estate and Business Brokers Act (REBBA). This is because making such guarantees is misleading and creates false expectations for clients. Real estate markets are influenced by a multitude of factors that are often beyond the control of a registrant, such as economic conditions, changes in zoning, and market demand.

Other actions, such as refusing to cooperate with another broker, may raise ethical questions but do not specifically violate REBBA guidelines in the same way. Providing inaccurate information about a property's zoning could be considered a violation due to the potential for causing harm or confusion, but it does not inherently involve a promise of future outcomes like option B does. Charging a commission rate below the industry average is permissible and can be a competitive strategy, thus it does not conflict with REBBA guidance.

Overall, guaranteeing an increase in property value misrepresents the nature of real estate investments and the inherent risks involved, making option B the correct answer in this context.