A undertakes to collect rents for B. B sues A for not collecting rent due from C to B. A denies that…

Section 17 makes statements by a person whose position or liability must be proved against a party to the suit admissions, if they would be relevant against that person in a suit concerning that liability. The illustration to section 17 says C's statement that he owed B rent is an admission and is relevant against A when A denies that rent was due, so the answer is "An admission, and a relevant fact as against A". C need not be a party, and the statement is used against A, not merely against C.