What is the main purpose of Safe Altitude 100 NM?

The main purpose of Safe Altitude 100 NM is to ensure clearance above all obstacles within 100 nautical miles of an aerodrome. This altitude is crucial for maintaining safety during flight by preventing aircraft from unintentionally colliding with terrain, buildings, or other structures that may be within that range of the airport.

This altitude benefits pilots by providing a vertical safety buffer when they are en route, especially during phases of the flight like departure and arrival, where terrain awareness is critical. The designation of this altitude goes beyond just accommodating obstacles directly surrounding the airport; it serves to account for any increase in elevation of the ground or man-made structures that could pose a hazard within that significant radius.

The other options do not capture the full scope of this purpose. For instance, while clearance during climb is necessary, it does not encompass the principle of clearance above obstacles in a wider area. Likewise, guidance for mountainous approaches or setting cruising altitudes are relevant to specific flight operations but are not the primary focus behind the designation of Safe Altitude 100 NM. The emphasis here is on broad safety and obstacle clearance, making option B the most accurate response.