Why does the length of the spring is going to increase in this image? Edit:
Original problem is,

In the following picture, length of sides of the cube is $10cm$ and its mass is $500g$. We attached a spring to its end with the spring constant of $50 \frac{N}m$. If the pressure of the cube at the bottom is $400 pa$. What is the change of the length of the spring from its original length?
$1)2cm \quad\text{decrease}\qquad
\\2)2cm \quad\text{increase }\qquad\\3)4cm \quad\text{decrease}\qquad\\ 4) 4 cm\quad\text{increase}$


Here, we are given that $F_{net}=4 N$ downward, hence we conclude the normal force is $1 N$ (denoted as black color). But then the teacher that was explaining this problem said the spring will be pulled downward because of the Newton's third law the force $1 N$ (denoted as green color) pull the spring downward and the length of the spring will be increased.
But I don't completely understood it. I think the normal force $1N$ cause the spring go upward and its length will be decreased (it is like someone press the end of the spring with their finger with $1N$ force and the length of the spring will be reduced).
What is the flaw in my reasoning?

 A: If the spring pulls upwards in the block with 1 N, then the block pulls downwards in the spring with 1 N. That is what Newton's third law tells. There is always an action/reaction force pair present.
And it is the downwards part of the force pair that in this case causes elongation.
Your drawing is slightly confusing since you have drawn some forces that act on the block and other forces that act on the spring, but not all. It is also not clear whether the green and black force arrows are located at the same point? To avoid confusion make two different diagrams: one with all forces that act on the block only and another diagram with all forces acting on the spring only.
A: Look at this way: Lets start with the cube sitting on the ground without a spring. The force on the ground is 5N (assuming that earth gravity is $10m/s^2$) and the pressure is 500 Pa.
Let's now attach a spring. If we were to push down on the spring (making it shorter in the process), the pressure on the ground would increase.
If you pull the spring up (making it longer) the pressure decreases until you have extended the spring by 10cm. At this point the pressure becomes 0 and the cube lifts off.
Since 400Pa are less than 500 Pa, we have clearly the second scenario here, so the spring length is increasing. The spring needs to provide 1N of force, the elongation is 2 cm.
A: Here is the force diagram which shows the force extending the spring

