What turbulence intensity causes changes in altitude and indicated airspeed but keeps the aircraft in positive control?

The correct answer is moderate turbulence intensity. In the context of aviation, moderate turbulence is characterized by changes in altitude and indicated airspeed that occur but do not impair the pilot's ability to maintain control of the aircraft. During moderate turbulence, the aircraft may experience a noticeable bumpiness, and occupants may feel changes in the aircraft’s vertical stability.

This turbulence level is typically manageable by pilots, as it requires some adjustments to maintain the desired flight path. However, it is significant enough to be felt and may cause brief interruptions to the flight operation.

In contrast, light turbulence usually does not cause any significant changes in altitude or airspeed; it is merely a slight, but it can still be noticeable. Severe turbulence is a much higher intensity where control can be significantly compromised, leading to more extreme fluctuations in altitude and airspeed. Clear air turbulence, while it can occur outside of storm conditions, does not categorize the intensity; it's a phenomenon rather than a measure of severity.

Thus, moderate turbulence is the best fit for the description provided as it encompasses the characteristics of changes in altitude and indicated airspeed while still allowing for positive control of the aircraft.