Which situation does not constitute a crime of unlawful imprisonment?

The situation where restraining a child by a relative for lawful purposes does not constitute a crime of unlawful imprisonment is based on the context of lawful authority and parental responsibilities. In instances where a relative, such as a parent or guardian, restrains a child to ensure their safety or to correct behavior, this action is generally considered permissible under the law, as it aligns with the authority that guardians have over minors for their well-being.

In contrast, the other situations described involve a lack of consent or the absence of lawful justification. For example, restraining an adult against their will or confining someone without their consent directly violates their autonomy and can constitute unlawful imprisonment. Likewise, restraining someone in a public space can also fall under unlawful imprisonment if it is done without justifiable cause or consent. Thus, option B stands out as it reflects an action taken within a recognized legal framework, signifying that it is not an offense.