Which tool is commonly used to assess a pet's heart rate?

The stethoscope is the appropriate tool for assessing a pet's heart rate because it allows the veterinary technician to listen directly to the heart sounds through the thoracic wall. This method provides an accurate count of the number of beats per minute and helps in identifying any abnormalities in the heart rhythm or sounds.

Other options, while related to veterinary care, serve different purposes. A thermometer is used to measure body temperature, which is significant, but does not provide information about heart rate. A sphygmomanometer is primarily used to measure blood pressure, not heart rate, and is generally more relevant when monitoring cardiovascular health in a comprehensive manner. The pulse oximeter measures the oxygen saturation in the blood and can give an indication of circulatory efficiency, but it does not directly measure heart rate in the way a stethoscope does. Thus, the stethoscope is the most relevant and effective tool for this specific assessment.