Question

which of the following devices directly measures the heart rate through electrical signals? wrist - based device, smart ring, smartphone app, chest - strap monitor

Explanation:

Chest - strap monitors are designed to directly measure heart rate through detecting the electrical signals of the heart (electrocardiogram - like principle). Wrist - based devices and smart rings usually measure heart rate through optical sensors (photoplethysmography). A smartphone app alone cannot directly measure heart rate through electrical signals without an external sensor.

Answer:

Chest - strap monitor