Step 1: Definition.
Misrepresentation is a false statement of fact, made innocently, that induces another party to enter into a contract.
Step 2: Analysis of statements.
(a) Correct — the false statement must be material to the contract.
(b) Incorrect — an unwarranted statement (without reasonable grounds) can indeed cause misrepresentation if it induces the contract.
(c) Correct — belief in truth does not prevent liability for misrepresentation.
(d) Correct — inducing a mistake as to the substance of the subject matter is misrepresentation.
Step 3: Conclusion.
Since (b) is contrary to the legal definition, it is the incorrect statement. \[ \boxed{(b)} \]