There was a serious . . . . . . . . between the two brothers.

Answer: Option B

Let's look at each option:

Option A: alliteration - Alliteration is the repetition of the same sounds, usually consonants, at the beginning of words. This doesn't fit the meaning of a serious disagreement.

Option B: argument - An argument is a disagreement or dispute. This fits perfectly with the sentence, describing a serious disagreement between two brothers.

Option C: alteration - Alteration means a change or modification. While there might be changes in a relationship after a serious disagreement, "alteration" doesn't directly describe the disagreement itself.

Option D: aberration - Aberration means a deviation from the normal or expected. This doesn't fit the context of a simple disagreement between brothers.

Therefore, the best word to complete the sentence is "argument" because it accurately describes a serious disagreement.