Answer: D. unable to talk during the activity.
Step-by-step explanation:
The talk test is a subjective method that is used for the estimation of appropriate cardiorespiratory exercise intensity. This method is used for maintaining the intensity of exercise at which the conversation of the person can be found to be comfortable.
When an exerciser reaches up to the intensity at which he/she barely respond in conversation then the intensity is considered to be safe and appropriate for the improvement of the cardiorespiratory endurance.
According to the able explanation, D is the correct option. The person will be unable to talk due to an increase in the intensity of the exercise.