By Fergal Smith and Nivedita Balu Dec 12 (Reuters) - Canada's main stock index fell on Tuesday as a drop in oil prices weighed on energy shares and investors assessed the latest U.S. inflation data for clues on the Federal Reserve's policy outlook. "The obvious culprit is the energy group and that's on the back of significant decline in oil prices," said Elvis Picardo, a portfolio manager at Luft Financial, iA Private Wealth. Cogeco CommunicationsCCA.TO fell 6% after Rogers Communications RCIb.TO sold its stake in the company to Canada's second-largest pension fund Caisse de depot et placement du Quebec.