Dot Spacing
This verbal description is of a speeding up motion. An object that speeds up over the course of its motion covers an increasing distance each successive second. So it is important that the dot separation distance increase as the object moves from the right towards the left.
Velocity Vectors
There are two important questions to answer when selecting the set of velocity vectors. First, which direction do they point? And second, is the length staying constant, increasing, or decreasing? The first question is easiest to answer. Velocity can be thought of as speed with a direction. The direction of the velocity is simply the direction that the object is moving; in this case, that is to the left. The second question pertains to the length of the vector arrow and that is where the speed comes in. A speeding up object is increasing its velocity; the magnitude or size is becoming greater over time. Thus, the length of the velocity vector will be increasing as the object moves from the left towards the right. This should narrow down the choices to one.
Acceleration Vectors
The first question to ask is: Is the object accelerating? Accelerating objects are either speeding up, slowing down or changing directions. This object is speeding up so it is accelerating. That rules out the choice of a zero acceleration.
The second question to ask is: Which direction is the acceleration directed? You must consider two things: (1) Which way is the object moving? and (2) Is the object speeding up or slowing down? An object that is slowing down will have an acceleration direction that is opposite the direction it moves. And if it is not slowing down (as in the case of this object), then the acceleration is in the direction that the object moves.
The third question to ask is: Is the length staying constant, increasing, or decreasing? The correct answer requires a thorough understanding of acceleration. First, acceleration is not velocity. An object can have a changing velocity but a constant acceleration. Acceleration has to do with the rate at which the velocity (or speed) is changing. If the change rate is the same each second, then the acceleration value is the same value from one second to the next. That is, the acceleration is a constant value if the amount by which the speed changes each second is a constant amount. Such instances would be represented by a set of acceleration vectors that are the same length. There's only two such choices.