Which of the following indicates a stall warning in an aircraft?

The indication of a stall warning in an aircraft is characterized by aural warnings accompanied by a feeling of control heaviness. When an aircraft is approaching a stall condition, various warning systems are activated, such as aural alerts that sound a beep or warning tone to alert the pilot of the impending stall. In addition to these auditory warnings, pilots typically experience a change in aircraft control responsiveness, often described as "control heaviness," which occurs due to the aerodynamic changes as the aircraft nears the stall angle of attack. This combination of aural alerts and control stick response is crucial for pilots to recognize the situation and take corrective action to avoid an actual stall.

The other options do not provide relevant indicators of a stall warning. For example, an increase in altitude does not relate to stall conditions and may occur under various flight circumstances. Maintaining a steady speed suggests stable flight conditions that are typically not associated with the onset of a stall. Similarly, the autopilot engaging indicates a specific flight mode engagement rather than a stall warning, and does not inherently provide information about aircraft performance or stall indications.