Final answer:
The word 'abuelo' correctly completes the sentence.
Step-by-step explanation:
The word abuelo correctly completes this sentence. In Spanish, abuelo means grandfather. In the given sentence, 'El padre de mi madre es mi _____', we are talking about the father of the mother, which is the grandfather. Therefore, the correct answer is abuelo.
Learn more about Completing a sentence in Spanish