Final answer:
The correct definite article to complete the sentence is "el".
Step-by-step explanation:
The correct definite article to complete the sentence is el.
Therefore, the complete sentence is: "Yo hablo con el hermano de mi amigo."
In Spanish, the definite article "el" is used before a masculine singular noun, in this case, "hermano." The definite article indicates that we are referring to a specific brother, in this context, of my friend.
Learn more about definite articles in Spanish