In what circumstance is it lawful to supply liquor to a minor?

The correct choice highlights the specific legal allowance in Ontario for supplying liquor to minors under controlled circumstances. In this case, a guardian giving liquor to a minor at a private residence is lawful as long as it is consumed there. This reflects the understanding that parents or guardians have the authority to supervise minors in a private setting, which includes the decision to allow them to consume alcohol under their watchful care. This is based on the belief that responsible adults can make informed decisions regarding alcohol consumption within their homes.

Other circumstances listed do not align with the legal provisions regarding minors and alcohol consumption in Ontario. Public places and events typically have stricter regulations to ensure the safety and well-being of minors, and parental approval outside of a private residence does not grant legal permission for serving alcohol to minors. Therefore, the context of a controlled, private environment, overseen by a guardian, remains the only lawful circumstance in this case.