Velocity is defined as the speed of an object in a specific direction. Among the choices provided:
- 35 m/s - This is a speed but lacks a direction, so it's not velocity.
- 15 km/hr west - This includes both a speed (15 km/hr) and a direction (west), so this represents velocity.
- 4 g/mL - This is a measure of density, not velocity.
- 12 m/s² - This is an acceleration, not velocity.
Therefore, the choice that represents velocity is 15 km/hr west.