Which category of ILS allows for the lowest visibility approaches?

The category of Instrument Landing System (ILS) that allows for the lowest visibility approaches is Cat 3. This system is designed to enable aircraft to land safely in very low visibility conditions, with minimum decision heights potentially down to zero for some subcategories. Specifically, Cat 3 includes different subcategories (such as Cat 3A, 3B, and 3C), each allowing for progressively lower visibility and decision heights, including autoland capabilities that require minimal input from the pilot during the landing phase.

In contrast, Categories 1 and 2 have higher minimum visibility requirements, which can restrict their use in specific weather conditions, making them less effective for low visibility situations. Category 4 does not exist within the standard classification and is not applicable. Hence, the correct answer succinctly identifies Cat 3 as the system allowing for the lowest visibility approaches.