Central banks across five continents are undertaking the broadest reduction in borrowing costs since 2009 to avert a global economic slump stemming from Europe’s sovereign-debt turmoil. The U.S., the U.K. and nine other nations, along with the European Central Bank have bolstered monetary stimulus in the past three months. Six more countries, including Mexico and Sweden, probably will cut benchmark interest rates by the end of March, JPMorgan Chase & Co. forecasts.