The correct answer is "الكسرة" because it is the strongest and most emphasized diacritic mark in Arabic grammar. It is represented by a small diagonal line placed above a letter and indicates a short vowel sound. The other diacritic marks, such as "الضمة" (dammah), "الفتحة" (fathah), and "السكون" (sukun), also represent vowel sounds but are not as strong or emphasized as the "الكسرة."