When an air traffic controller wants to be certain that a pilot has received an important transmission, the controller may append the word “acknowledge” to the instructions being issued. Student pilots should note that a similar procedure has been added to the stall-recovery demonstrations required by the Private Pilot-Airplane Airman Certification Standards. Part of the demonstration of skills associated with power-off and power-on stalls is to “acknowledge cues of the impending stall and then recover promptly after a full stall has occurred.”
In the context of pilot/controller communications, a controller’s request that you acknowledge a transmission translates to, “Let me know that you have received and understood this message,” as the term is explained in the Pilot/Controller Glossary.
The Airplane Flying Handbook explains in Chapter 4 that sensory cues may include feel, vision, hearing, or kinesthesis (defined in Chapter 3 as “the ability to sense movement through the body”).
Different stalls may produce different cues. “In a power-off stall, for instance, the cues (buffeting, shaking) are less noticeable than in the power-on stall. In the power-off, 1G stall, the predominant cue may be the elevator control position (full up elevator against the stops) and a high descent rate,” the handbook notes.
As for how to make the verbal acknowledgement, it can be as simple as “announcing ‘stall warning’ or ‘buffet,’” wrote designated pilot examiner Bob Schmelzer in the November 2017 Flight Training article, "Checkride: A different approach."
Don’t fixate on the new requirement to vocalize your aircraft’s condition and become distracted from performing the rest of the stall task, including recovering promptly after a full stall has occurred, using procedures in your pilot’s operating handbook; retracting flaps to the recommended setting; retracting the landing gear, if retractable, after a positive rate of climb is established; and accelerating to Vx or Vy “before the final flap retraction” and returning “to the altitude, heading, and airspeed specified by the evaluator.”