Choosing a relative pronoun
Choose carefully between that, which, and who.
As a general rule, use that when a relative clause is restrictive (without a comma), which when a relative clause is non-restrictive (with a comma), and who in either case whenever the antecedent is a person.
This is the main problem, which we must solve.
This is the main problem that we must solve.
She was wearing a coat, that was sky-blue.
She was wearing a coat, which was sky-blue.
I know a man that owns twenty horses.
I know a man who owns twenty horses.