Which fiduciary duty did Lionel violate by delaying communication of a counteroffer?

Lionel violated the fiduciary duty of due diligence by delaying communication of a counteroffer. This duty requires real estate agents and brokers to act with care and promptness in all dealings to protect their clients' best interests. In the context of real estate transactions, timely communication is critical. If an agent delays in communicating important information, such as a counteroffer, it can negatively impact the client's ability to respond effectively to offers or negotiate terms.

In this scenario, Lionel's delay could have deprived his client of opportunities or caused a loss in negotiating power. Due diligence encompasses proactively advocating for a client and ensuring that all critical developments are communicated in a timely manner. Maintaining the momentum of negotiations and ensuring that the client is informed allows them to make well-informed decisions, which is a key aspect of fulfilling the responsibility of due diligence.

The other options, while relevant to fiduciary duties, do not apply in this situation as directly as due diligence.