Do you seek advice from others when you have a question? Will you evaluate their inputs before you consider their advice?
A question such as this is asked by interviewers with a goal of determining an individual's preference in terms of who to look up to as their role models especially in decision-making issues. Just as you are required to mention a list of character references on your resume, your interviewer will also be interested to know the people whom you highly regard when it comes to making choices. The people you mention (who can be your mentors) are representations of who you would like to be. You need to choose these individuals well.
Since your answer will be used as a gauge of your qualifications, an intelligent answer with well-supported explanation is sure to strike a good impression from your interviewer.
Since the question asks "Who", it simply seeks for a specific individual (or you can name a few if you wish) whom you personally perceive as an example in making important decisions. Start your response by naming the person/s and then giving a brief description of who the person is in relation to you, what the person does, the individual's accomplishments, and why this person has created such an impact in your life apart from the aspect of decision-making. In your description, mention if you have personally worked with the person or if he or she is just one of those you constantly interact with apart from the work setting.
An effective response to this question is to mention only the accomplished people who have some sort of relation to you whether in the personal or professional aspect. You might want to mention your former boss in your previous employment, or your parents whom you have always relied upon for the best advice, or your closest office colleague who has been your personal advisor when faced with dilemmas.
In your response, it is important to limit only the number of individuals to one or two. Remember that the question focuses on your personal decision making; mentioning quite a few names will only give the impression that you simply place your trust on anybody or anyone who you get to converse with.
The most important point in your response to this question is how you assess the inputs of your mentors. If you want to make an impression, never mention that you are reliant on other people's decisions for you. It simply denotes that you are not able to stand on your own feet when facing a challenging situation or when a problem arises. The best way to address the follow-up question is to emphasize to your interviewer that you are simply seeking for advice, suggestions, and opinions of trusted people in your life but the bottom line is, you make the wise decision yourself. Create the impression that you know how to weigh things based on need and importance.