Velocity is defined as the speed of an object in a given direction. Among the options provided, the one that represents velocity is:
15 km/hr west
This choice includes both a speed (15 km/hr) and a direction (west), making it a measure of velocity. The other options represent speed (35 m/s), density (4 g/mL), and acceleration (12 m/s²), which do not qualify as velocity.