Step-by-step explanation:
J and K would both be incredibly biased towards one side or the other; the president of the wind-energy company would be biased against it, while the owner of the coal-mining company would be biased towards it.
This leaves a toss-up between L and M. A research writer's job is to gather information and sources for a story, whereas a columnist does research and releases an opinion piece based on said research. The introduction of an opinion automatically introduces bias, thus making the research writer the least biased/most balanced option.