Concept:When a group of people share a mutual action, we use a reciprocal pronoun like
each other.
Explanation:The four brothers are doing the same action to one another: each brother has not seen the other brothers.
A reciprocal pronoun expresses this mutual relationship.
Each other is used for two or more people acting toward one another.
Example: The two friends helped each other. This means each friend helped the other.
Here, the subject is "the four brothers", and the missing word should show mutual seeing.
Themselves is a reflexive pronoun, used when the subject and object are the same people, meaning they have not seen their own faces, which is not the intended meaning.
Each another is not a correct reciprocal phrase; we use "each other" or "one another".
Theirself is not a standard English word.
Therefore, the correct option is
each other.
Answer:D. each other