What factor determines the need for anti-icing systems during period of frost formation?

The need for anti-icing systems during periods of frost formation is primarily determined by the wing bulk fuel temperature at and below 0°C. Frost can significantly impact an aircraft's performance by altering the airflow over the wings, which can lead to reduced lift and increased drag. When the wing bulk fuel temperature falls to or below freezing, there is an increased risk of frost forming on the aircraft surfaces, necessitating the activation of anti-icing systems to ensure safe flight operations.

In contrast, the other options do not directly correlate with the fundamental requirement for anti-icing systems. For example, ice build-up on windows, while a concern for visibility, does not impact the aircraft's aerodynamic performance in the same critical way that frost on wings does. High altitudes alone do not dictate the need for anti-icing systems without considering temperature conditions, and clouds within 2000 feet above ground level are not specifically a factor that determines frost formation related to the need for anti-icing systems. These points emphasize why the wing bulk fuel temperature is the crucial factor in this context.