Over the summer, as the U.S. stock market wobbled and then plunged 4 percent in a day, Citigroup made a tough choice: to dial down risk-taking in the bank’s capital markets group. Phil Drury and Doug Adams, then co-heads of U.S. equity capital markets focused on block trading, a corner of the trading business where razor-thin margins can quickly turn to big losses when markets sour, according to people close to the matter. Citi plunged in the rankings for that part of the market during the third quarter to number eight, according to data provider Ipreo.