Based on lines 1-10 of the poem "Knock Knock," the most accurate description of the father and son's relationship would be:
They are close and have fun together.
This response suggests a positive bond where they likely enjoy each other's company, which is commonly reflected in playful or affectionate language typical of such relationships. The other options imply some distance or lack of familiarity, which doesn't align with the depiction of their interaction in those lines.