Which of the following best describes an "open house party"?

An "open house party" is best described as a social gathering at a residence where hosts invite guests to come and go freely throughout the event. This type of gathering typically encourages a relaxed atmosphere, allowing friends, family, and acquaintances to socialize in a home setting. Open house parties often take place to celebrate special occasions, holidays, or simply to bring people together for casual interaction.

The other options do not accurately capture the essence of an open house party. A formal gathering organized by a business implies a more structured, professional event with specific purposes and often a guest list, which does not align with the informal nature of an open house. Similarly, an official state event suggests a government or formal occurrence that follows protocol, which contrasts with the personal and informal vibe of an open house. Lastly, a casual meetup in a public space refers to gatherings outside of a home environment, lacking the residential context that defines an open house party.