The Cigna Group (NYSE:CI) is in advanced talks to sell its Medicare business in an about-face for the health-insurance giant, which had been expanding its footprint in the fast-growing sector. Cigna, which has been running an auction for the business, known as Medicare Advantage, is now in exclusive talks to sell it to Health Care Service Corporation, a Mutual Legal Reserve Company for between $3 billion and $4 billion, according to people familiar with the matter.