Final answer:
The form of the argument 'If p, then q. p. Therefore, q.' is known as modus ponens, a fundamental structure in deductive reasoning.
Step-by-step explanation:
The form of the argument presented, 'If p, then q. p. Therefore, q.' is called a modus ponens. This is a valid deductive inference, where if the first premise 'If p, then q' is understood as p being sufficient for q, and the second premise confirms that p is true, then the conclusion that q must be true logically follows. The structure of modus ponens is critical in understanding valid deductive reasoning.