Explanation

First find the vertical height: h = √(l² − r²) = √(13² − 5²) = √(169 − 25) = √144 = 12 cm. The volume of a right circular cone is (1/3)πr²h = (1/3) × π × 25 × 12 = 100π cm³. The (1/3) factor is essential — dropping it gives the tempting distractor 300π, and using the slant height as h is another classic error. Hence option (A) 100π cm³.