A controller must ensure that a departing aircraft can safely _______ without conflict.

The correct choice emphasizes the importance of a departing aircraft being able to safely execute a standard climb without encountering any conflicts with other air traffic in the vicinity. During departure, pilots are trained to follow specific climb profiles that take into account the performance of the aircraft and surrounding traffic patterns. It is crucial for air traffic controllers to ensure separation between departing and arriving aircraft, as well as any other traffic in the airspace. By ensuring that a departing aircraft can safely execute a standard climb, the controller is facilitating the aircraft's transition into the airspace while maintaining safety margins.

Looking at the context of other options, crossing the runway is more apt for aircraft that have landed or are taxiing and does not directly pertain to a departing aircraft's climb. Arriving at an altitude is generally part of the departure procedure, but it doesn't encapsulate the entire aspect of ensuring safety during the climb phase, which involves more than just reaching altitude. Lastly, navigating through the approach is of greater relevance to arriving aircraft rather than departing ones, hence it's not the primary concern during a departure. This focus on a standard climb reflects the need for safe and efficient traffic management in the terminal airspace.