The best known Hawthorne study was that of lighting-level effects by Roethlisberger and Dickson (1939). In this study employees were taken to a special room where lighting levels were varied from day to day to see how performance would be affected. They found that

They found that workers were more responsive to social factors such as the people they worked with and the amount of interest their manager had in their work.

Explanation:

The Hawthorne study involving lighting level effects was carried out to  determine motivating factors behind productivity at work. Four studies were used in this experiment;

In the first study, across three departments, increased productivity was observed whether or not the illumination was increased.In the second study, the first group had the lightening unchanged, while the second group had the lighting increased. There was also, an increased production in both groups, just as was observed in the first study. In the third study, a decrease in lighting was observed. two groups, one group having stable lightening and the other, having decreased lighting were studied. There was again, increased productivity in both groups until the decreased lighting got really low and the production fell.In the fourth study, two women were observed and upon varying lighting levels, there was maintained constancy in production

The study therefore, didn't yield results on productivity in response to lighting. It was then seen that social factors, were the biggest motivator in job performance, mainly how much attention was being paid to individual employees. Being knowingly watched, was seen to have a positive effect on employee productivity.

It's most likely choice a, it makes the most sense in my opinion.