Answer:
Miguel es el menor de los muchachos.
Step-by-step explanation:
Miguel is the youngest of the three kids. He's five, Pepe's six and Julio is ten years old. Mayor means oldest, so the first sentence is not true. Neither is the second since Julio is older than Pepe. Julio is, in fact, the eldest. So the third sentence is also wrong. "Menor" can also be referred to as "más joven" o "más pequeño" when talking about kids.