Explanation

From tan 6θ · tan 3θ = 1 we get tan 6θ = cot 3θ = tan(90° − 3θ). Hence 6θ = 90° − 3θ + 180°n, i.e. θ = 10° + 20°n. In the range 0 < θ < 30° the only admissible value is θ = 10°, option B.