My uncle has three sons, --- work in the same office.

Explanation: The correct phrase is 'All of whom,' which is grammatically appropriate and relates to the subject. Correct: D. Incorrect options: A. 'Who all' is informal and ungrammatical. B. 'All of them' does not connect properly to the clause. C. 'They all' forms an incorrect structure. Note: Relative pronouns like 'whom' are essential for forming clauses that refer back to a subject in formal contexts.