The correct option is (C) 1280 m.
Explanation:
Area of circular region = πR2 = π × (128)2 = 51.47 × 103 km
We know area of transmission region = π(d2) = π(√2hR)2 = 2πRh
∴ Height of antenna = h = (area / 2πR)
= [(51.47 × 103) / (2π × 6400)]
= 1.2799km
= 1280m