If you want to intercept the 300 magnetic course outbound from the selected NAVAID as shown in Fig 8, what action should you take?

To intercept the 300 magnetic course outbound from the selected NAVAID, maintaining your present heading is the appropriate action if your current heading aligns with the desired course. This means that the aircraft is already on a trajectory that will allow it to track directly to or maintain the established course without needing to make an immediate turn.

In the context of navigation and intercepting courses, pilots must consider their current position and heading relative to the desired course. If the aircraft is already aligned with the magnetic course of 300 degrees, there is no need for a course correction. Thus, maintaining the current heading allows for a seamless transition onto the desired track.

The other options imply unnecessary maneuvers. For instance, turning left immediately could cause the aircraft to stray from the intended course, climbing to a higher altitude may be irrelevant to course interception unless specified by terrain or ATC, and beginning a descent does not inherently assist in maintaining or intercepting the specified magnetic course. Each of these alternatives could lead to increased complexity and potential navigation errors if the course is already being correctly followed.