What is defined as a response to a plaintiff's claim that does not deny the plaintiff's facts but attacks their legal right to bring an action?

The concept described is an affirmative defense. An affirmative defense is essentially a statement by a defendant that, while they may not dispute the facts of the plaintiff’s claim, there are legal reasons that justify or excuse the defendant's behavior. This means that the defendant asserts that even if the plaintiff's facts are correct, the law does not allow the plaintiff to prevail in their lawsuit.

For example, in a situation where a plaintiff sues for breach of contract, the defendant might acknowledge that a contract existed but argue that it was void due to a lack of capacity or legality. This does not deny the existence of the contract but rather hinges on legal principles that negate the plaintiff's right to recover despite the alleged facts being accurate.

The other choices relate to different legal concepts: a counterclaim is a claim brought by a defendant against a plaintiff, an injunction is a court order limiting or requiring certain actions, and a motion to dismiss is a request to terminate a case based on specific legal deficiencies. These choices do not specifically address the scenario where the defendant accepts the facts but contests the legal grounds for the plaintiff's action, which is why they do not fit the definition provided in the question.