(a) is fine, (b) is fine grammatically, but less common since one would tend to use a pronoun like "who" or "whom" for a person, and "that" or "which" for a non-person.
This is not a strictly enforced "rule", and it is easy to find examples of "a person that" or
We are a small business with five employees, one of which is looking to become pregnant. This would cripple us as a business financially. What are our rights and what can we do about this? (source)
(By the way, although this gives an example of "which", the legal advice given at that site is horrid - it suggests finding an excuse to fire an employee who is pregnant, and that is simply illegal)