Investment banks often have conflicting roles. They sometimes act for

Start with the conclusion: "Analysis of companies’ financial health need to be unbiased if an investment bank is to achieve long-term success."

Now, we need to find a practice that, if adopted by an investment bank, would hinder its long-term success. In other words, we need to select a practice that would cause the investment bank's analysis of companies' financial health to be biased, since that would hinder its long-term success.
The passage tells us that investment banks "sometimes act for a client company by raising capital from other investment institutions as advantageously as possible". Choice B simply describes one practice that the investment bank could use to determine how best to raise capital for a client. Certainly, the bank could use reports by its analysts to determine how best to raise capital for a client while simultaneously providing unbiased analyses of companies' financial health. This practice alone does not necessarily lead to bias.
If a manager is solely engaged in raising capital for clients, then that manager would certainly not want other clients wishing to make investments to see "unfavorable reports on the financial health of companies for whom they are raising capital." Those managers would only want the clients making investments to see favorable reports and would be more likely to give positive recommendations for analysts who send favorable reports. Because analysts want positive recommendations from those managers, the analysts would be more likely to send favorable reports. Thus, the analysis by those analysts would be biased, and, according to the author, this would hinder the bank's long-term success.