Final answer:
When an aircraft cannot meet the climb gradient requirements specified under FAR Part 135, it cannot legally depart, as the aircraft's ability to safely ascend to clear obstacles, especially during an engine failure, must not be compromised.
Step-by-step explanation:
The question pertains to the climb gradient requirements in aviation, specifically under FAR Part 135. The climb gradient is a critical parameter that ensures an aircraft can safely ascend to clear obstacles after takeoff, even with an engine failure. When an aircraft cannot meet the specified climb gradients due to its performance limitations, the minimum weather conditions become a key consideration for legal departure. To address this scenario, let's consider the standard instrument departure procedures and obstacle clearance requirements that come into play.
However, based solely on the options provided and without reference to specific regulations or scenarios that might allow for alternative minimums, the general rule in aviation is that if the aircraft cannot meet the climb gradient, then you cannot legally depart, regardless of the visibility or ceiling conditions. Safety protocols demand that the aircraft's ability to climb, particularly in emergency situations, is not compromised.