When can an aircraft be cleared to fly through class B, C, D, or E airspace?

An aircraft can be cleared to fly through Class B, C, D, or E airspace when it has a visibility report of 1 statute mile or more, which is a requirement for VFR (Visual Flight Rules) operations. This visibility standard helps ensure that the pilot can see and avoid other aircraft and obstacles, which is essential in more congested airspace where there is a higher likelihood of interaction with other air traffic. Visibility is a crucial factor in maintaining safety and situational awareness for pilots navigating through these controlled airspaces.

The other options involve considerations that are not primary criteria for clearance. While being on a scheduled flight may aid in routing and prioritization, it does not alone grant access to controlled airspace without meeting visibility and other safety standards. Departing from an uncontrolled airport can happen without needing to be in specific controlled airspace, and it doesn’t inherently provide clearance for Class B, C, D, or E. Lastly, the presence of other flights in the vicinity is a context-dependent safety consideration, but clearance is not automatically contingent upon whether or not other flights are present; it is primarily based on the pilot's ability to comply with visibility and communication requirements.