Answer:
A.
Step-by-step explanation:
When two objects are moving at constant velocities in the same direction, the relative velocity is simply the difference between their velocities. In this case, you are traveling at a velocity of 90 km/h and the other car is traveling at a velocity of 120 km/h. Therefore, your velocity relative to the other driver is 120 km/h - 90 km/h = <<120-90=30>>30 km/h.
The correct answer is therefore 30 km/h, which corresponds to choice (a). The other choices are incorrect because they either have the wrong magnitude or the wrong direction. For example, choice (b) has the correct magnitude but the wrong direction, while choice (c) has the correct direction but the wrong magnitude. Choice (d) has the wrong magnitude and the wrong direction.