Answer: B
Step-by-step explanation:
Notice the pull on both sides of the rectangle block.
For the first diagram, notice that the pull is 10N on both sides. If the force is the same on both sides, then the net force is 0N because there is no difference on each side.
For the second diagram, notice that the pull is 10N on the right side two sides. That means all the force is being pushed to the right side, which totals to 20N.
For the third diagram, notice that the pull is 10N on the left side and 8N on the right side. This means the both sides balance out, but with 2N extra on the left side.