Answer: From the given comprehension if the following statement is true, will most seriously weakens the argument: Among the entrepreneurs surveyed, those who did not produce formal business plans sought and received a much larger proportion of their capital from investors with whom they had a long-standing business relationship.
Here it states that, entrepreneurs who were capable of raising funds without thought/design were able to make the capital because of their relation with investors.
Therefore, the correct option here is (c).