Final answer:
During the first phase of blood pressure measurement, the sounds heard are called Korotkoff sounds, typically experienced as thumping sounds, which indicate systolic pressure.
Step-by-step explanation:
The sounds heard during the first phase of blood-pressure measurement are Korotkoff sounds. These are the turbulent sounds heard with a stethoscope when the cuff pressure of the sphygmomanometer is reduced and blood flow begins to return to the arm. The first sound, which is typically a thumping sound, indicates the systolic pressure. These sounds are different from heart murmurs, which are unusual sounds caused by turbulent blood flow due to heart valve issues. Therefore, the correct answer to the student's question is C. thumping sounds.