the bottom right graph is correct. x must fall between 3 and 6, so that automatically rules out the bottom left, as one of the points falls on 2.
the top left is incorrect because it doesn't show that x is greater than or EQUAL to three. it has an open circle rather than a closed circle. that's two answer choices eliminated.
from there, you just have to refer back to your given information:
3≤x≤6
the given info says that x must be between or equal to 3 and 6. the top right graph leaves out a "step" because it has 5 and 6 as solid dots side by side.