Velocity is a vector quantity that includes both speed and direction. Among the choices provided, the one that represents velocity is:
15 km/hr west
This is because it specifies both the speed (15 km/hr) and the direction (west). The other options either represent speed (35 m/s) or are not related to velocity (12 m/s², which is acceleration, and 4 g/mL, which is a density).