Final answer:
King Frederick William IV responded to the demonstrations in Berlin by refusing to accept the constitution proposed by the representatives of the Frankfurt Assembly.
Step-by-step explanation:
King Frederick William IV responded to the demonstrations in Berlin by refusing to accept the constitution proposed by the representatives of the Frankfurt Assembly. He called the offered position a 'crown from the gutter' and rejected it. This refusal by King Frederick William IV and the opposition from the aristocracy and military hindered the unification of Germany under Prussian rule at that time.