Final answer:
A licensee with plumbing knowledge should disclose a plumbing defect to the buyer, as it is mandatory to inform potential buyers of all known defects.
Step-by-step explanation:
Upon discovering a plumbing defect not listed on the Real Estate Condition Report nor identified in the home inspection report, a licensee with relevant background knowledge should disclose the defect to the buyer. Ethical and legal standards in real estate require that all known defects be disclosed to potential buyers to ensure that they can make informed decisions. Ignoring the defect (Option B) is neither ethical nor legal, confronting the seller (Option C) is not a direct solution for the buyer's benefit, and seeking legal advice (Option D) should not preempt the immediate obligation for disclosure.