ANSWER:
In order to obtain an unbiased sample for the purpose of this type of research, the most appropriate option would be option C.
(C) Place suggestion boxes at random locations in the company's plant and offices.
Note that, the other suggested options would yield results but these might not be free from bias since it does not guarantee anonymity. However, with the use of option C, each participant in the survey can protect their identity as much as he/she chooses.