A - Companies started by entrepreneurs who had used formal business plans to attract investment were on the whole as profitable in their first year as were companies started by entrepreneurs who had not produced such plans.
Out of context as we are talking of securing the capital and not profitability. Even if it were within the context, it may just be strengthening rather than weakening the arguement as the profitability in both cases is comparable

B - In surveys of entrepreneurs who have attempted without success to raise sufficient capital, more than half of the respondents indicate that they have produced a formal business plan.
It rather strengthens the argument , in which it says that there is an equal chance of failing even after making formal plan.

C - 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.
CORRECT... this proves that entrepreneurs who have been able to raise funds without a plan were able to make up the capital due to their relationship with investors.

D - The entrepreneurs surveyed who did not produce a formal business plan spent nearly as much time doing preparatory research and planning as the entrepreneurs who produced plans.
Out of context .. we are not talking of time

E - The entrepreneurs who produced business plans generally reported later that the process of writing the plan had increased their confidence that their company would succeed.
Again out of context

Answer: C