Explanation:

Frame of reference:
A frame of reference is a coordinate system that is used to describe the motion of an object. It is a set of axes or a system of coordinates in relation to which the position, velocity, and acceleration of an object can be described.

Relative frame of reference:
A relative frame of reference is a coordinate system that is moving with respect to an observer. It is a frame of reference that is attached to an object or observer and moves with it. The observer measures the motion of other objects relative to their own frame of reference.

Absolute frame of reference:
An absolute frame of reference is a coordinate system that is fixed and does not move with respect to an observer. It is a frame of reference that is fixed in space and does not change its position or orientation. The observer measures the motion of other objects relative to this fixed frame of reference.

Ground frame of reference:
The ground frame of reference is a specific type of absolute frame of reference that is fixed to the Earth's surface. It is commonly used to describe the motion of objects on the Earth's surface, such as cars, pedestrians, etc.

Explanation of the correct answer:
In this question, the observer is sitting on a car that is moving with some constant velocity. The observer sees things around him, which means he is observing the motion of objects relative to his own frame of reference. Since the observer is in the car and the car is moving, his frame of reference is a relative frame of reference. Therefore, the correct answer is option 'A' - Relative frame of reference.

The observer measures the motion of objects around him relative to his own frame of reference, which is the moving car. For example, if the observer sees a tree passing by him, he would describe the motion of the tree relative to his own frame of reference (the moving car). The observer would perceive the tree as moving backward if the car is moving faster than the tree, or as moving forward if the car is moving slower than the tree.

It is important to note that the observer's frame of reference is relative to him and may be different from other observers who are in different frames of reference. Each observer would measure the motion of objects relative to their own frame of reference, depending on their own motion or lack thereof.

In conclusion, the observer sees things around him in the relative frame of reference because he is sitting on a car that is moving with some constant velocity.