All the options provided are correct grammatically. However, the difference lies in the capitalization.
Option A: The capitalization for "Grandparents" and "Great-grandfather" is incorrect.
Option B: The capitalization for "grandparents" and "great-grandfather" is correct, as these words are not used as proper nouns.
Option C: The capitalization for "Grandparents" is correct, but "Great-grandfather" should not be capitalized unless it is part of a specific name.
Option D: The capitalization for "grandparents" and "great-grandfather" is correct, as these words are not used as proper nouns.
Both options B and D are grammatically correct and have appropriate capitalization.