Step 1: Read for meaning. Mei Ling is responsible for her sister while their parents are away — this means she is taking care of her. Step 2: Match meaning to phrasal verb. 'Look after' = to take care of someone. 'Look for' = to search. 'Look up' = to search for information. 'Look at' = to direct gaze. Step 3: Select the correct preposition. Only 'after' gives the meaning of caring for someone. Answer: after