Answer:
Option D is correct - The Harappan civilization fell to Aryan invaders.
Explanation:
Option A is not correct as we were not able to decipher their letters yet.
Option B is not correct as Harrapan civilization didn't managed to develop outside the borders of Ancient India, while Egyptian civilization affected many regions, including Middle and Near East.
Option C is not correct because the cities of Harrapan civilization were regarded as the largest and most developed cities of the time.
D is correct as after the collapse of civilization invaders from the North came and started a new period, known to us as Vedic period.