Answer:
C. biased, understating the effectiveness of the diet.
Step-by-step explanation:
As the company promises the population of America which is too huge, just on the study based on 20 employees of the company itself.
This clearly means that the company is trying to sell the product with false reports as the sample size of study is to small to represent entire American Population.
Further that too the employees could be influenced to get the false results.
As since the employees could be influenced and that the results can be altered accordingly, the report is biased, and is misleading.