Looking at the graphs, we have the following analysis:
I - The points look like a line, so they suggest a linear relationship.
II - The points also look like a line, so they suggest a linear relationship.
III - The points are very separated, they don't form any pattern, so they don't suggest a linear relationship.
IV - The points go up and then down, suggesting a quadratic relationship (like an arc), so they don't suggest a linear relationship.
Therefore the correct option is C: III and IV only.
