Hint: Recall the concept of uniform motion and uniformly accelerated motion.

Step 1: Find the nature of the displacement-time graph in uniform motion.
When we are calculating the velocity of a displacement-time graph we have to take the slope. Similarly, we have to take the slope of the velocity-time graph to calculate acceleration.

Step 2: Find the nature of the velocity-time graph for uniformly accelerated motion.
When the slope is constant motion will be uniform. When we are representing motion by a graph it may be displacement-time, velocity-time or acceleration-time hence, B may represent time. For uniform motion, the velocity-time graph should be a straight line parallel to the time axis. For uniform motion velocity is constant hence slope will be positive. Hence quantity A is displacement.
Therefore, for uniformly accelerated motion slope will be positive and A will represent velocity.

Hence, option (3) is the correct answer.