The Sensex at the Bombay Stock Exchange gained 159 points, or 1 percent, to 15,858, on Thursday, on fresh buying by investors, amid the settlement of the monthly derivative contracts, a fall in food inflation, and rising Asian and European markets. The Nifty at the National Stock Exchange rose 50 points, or 1.1 percent, to 4,756.Stock markets in China, Hong Kong, South Korea and Singapore rose between 0.1 percent and 0.7 percent.