Which among the following was not recognized as a major dialogue of Plato?

The correct answer is Metaphysics.

Plato, one of the most influential philosophers in Western history, wrote his philosophical works primarily in the form of dialogues, where characters (often featuring his teacher, Socrates) engage in conversation to explore complex ideas. Among his most famous and widely recognized dialogues are Crito, which discusses justice, injustice, and the obligation to obey the laws, and Phaedo, which details the final hours of Socrates and explores the immortality of the soul.
In contrast, Metaphysics is not a dialogue by Plato. It is actually a seminal treatise written by his most famous student, Aristotle. In this work, Aristotle investigates the nature of existence, reality, and first principles, distinguishing it completely from Plato's dialogic body of work.