Answer:
The answer is: C) "Dear Senior Manager of Human Resources"
Explanation:
If Brian uses the expression "Dear Sir" or "Dear Madam" he will be displaying gender bias, by assuming the gender of the senior manager. If he just says "Hi" he wouldn´t be considered very polite and formal.
The best way he can address the person is by just naming the position without regarding whether a man or a woman is the manager. That way he will be seen as both polite and formal.