(c) 6.4m/s2 .
Explanation:
When the moon strikes the earth its center is R/4 above the surface of the earth. So the acceleration of the moon will be calculated at R+R/4 = 5R/4 away from the center of the earth. Since the acceleration is inversely proportional to the square of the distance, the acceleration of the moon at the time of strike
= g*{R/(5R/4)}² = g*(4/5)² = 16g/25 =160/25 =6.4 m/s².