The correct answer is A. impartial.
An impartial person is someone who does not favor one side over another. They are fair and objective in their judgment.
A biased person is someone who favors one side over another. They are not fair or objective in their judgment.
A neutral person is someone who does not take sides. They are not biased towards either side.
A prejudiced person is someone who has a negative opinion about a particular group of people. They are not fair or objective in their judgment.
In the given question, the person is described as being very careful and never taking sides. This suggests that they are impartial. They are fair and objective in their judgment, and they do not favor one side over another.