Final answer:
The signal for systolic pressure during the Blood Pressure Skill is marked by the first Korotkoff sound heard through the stethoscope.
Step-by-step explanation:
When performing the Blood Pressure Skill, the signal that indicates the systolic pressure is a. The first Korotkoff sound. This occurs when the cuff is deflated enough for blood to just start flowing through the constricted brachial artery, producing the first audible sound through the stethoscope. The diastolic pressure is indicated by the last Korotkoff sound, which is the point at which these sounds disappear, indicating that blood is now flowing freely through the artery without any restriction from the cuff.