An advertisement for an apartment building states it is "adult only." This advertising is:

The advertisement stating that the apartment building is "adult only" can be legal if the property qualifies as housing for older persons under federal law. Specifically, the Fair Housing Act allows for certain housing developments to limit occupancy to individuals above a certain age, provided that at least 80% of the units are occupied by at least one individual who is 55 years of age or older.

This provision was created to promote and maintain communities specifically designed for older adults, while also ensuring protections for those communities to offer a certain lifestyle not accommodating children. When a development meets the criteria set forth in the Fair Housing Act, it can legally advertise itself as being for adults only, thus making this option correct.

Moreover, while the other responses address various considerations regarding housing discrimination, they don't accurately describe the legal nuances surrounding age-restricted housing. For instance, while some state laws may impose more stringent rules or protections against discrimination, the federal law does provide a clear pathway for adult-only housing when specific conditions, such as having a significant number of residents aged 55 or over, are met. Similarly, statements about married adults or outright discrimination against children do not align with the established legal framework for age-restricted housing under the Fair Housing Act.