An undercover state trooper posing as a food stamp recipient was able to draw cash using an electronic benefits transfer card at five Brockton businesses, Plymouth County prosecutors said Thursday. Eight people are facing charges after the sting operation. Officials said the inquiry, which began in May, uncovered a variety of scams involving misuse of the EBT cards, which are used to distribute food stamps and some cash benefits to the poor. Prosecutors said search warrants were executed at the B&D Mart on Montello Street; JP Communications on North Montello Street; and Lucky Dollar, Rice’s Market, and Dip N’ Sip, all on North Main Street. Eight individuals were arrested, prosecutors said.