Explanation:

In the given question, we are asked to identify the correct term for our father's father. The correct answer is option 'D' which is Grandfather. Let's understand why this is the correct answer.

Definitions:
- Grandfather: The father of a person's parent is called a grandfather. Grandfathers are the male ancestors in a person's family who are two generations above them.
- Cousin: A cousin is a person who is the child of one's aunt or uncle. Cousins are usually of the same generation as us.
- Aunt: An aunt is the sister of a person's parent. Aunts are typically the female siblings of our parents.
- Grandmother: The mother of a person's parent is called a grandmother. Grandmothers are the female ancestors in a person's family who are two generations above them.

Explanation:
- When we talk about our family tree, we have different terms to address our relatives based on their relationship with us.
- Our father's father is our paternal grandfather. He is the father of our father.
- The term "grandfather" is used to refer to him because he is two generations above us.
- He is someone who is related to us through our father's side of the family.
- It is important to note that the term "grandfather" can also be used when referring to the father of one's mother. He is called the maternal grandfather.
- Similarly, the mother of our father is called our paternal grandmother and the mother of our mother is called our maternal grandmother.

Conclusion:
- In conclusion, the correct term for our father's father is Grandfather. This term is used to address the male ancestor who is two generations above us and is related to us through our father's side of the family.