Under what circumstances does ATC issue a STAR?

Answer: B — When deemed appropriate by ATC.

A Standard Terminal Arrival Route (STAR) is designed to provide an efficient and safe transition for aircraft from the en-route phase of flight to the terminal area. ATC issues a STAR when it is deemed appropriate based on several factors including the traffic volume, weather conditions, and specific operational needs within the airspace. By having structured arrival routes, ATC can manage traffic more effectively, ensuring that aircraft are sequenced in a manner that optimizes spacing and minimizes delays. This decision is a part of ATC's responsibilities to maintain safety and efficiency in the airspace, and it can be influenced by current and anticipated traffic levels, airspace restrictions, or specific airport procedures. In summary, ATC evaluates the situation and determines when a STAR is appropriate, making it a proactive measure for traffic management rather than a reaction to specific requests or circumstances like busy periods or flight hierarchies.