Answer:
Mountain Ranges.
Explanation:
The natural boundary which shielded the Indus and Ganges Valleys from invaders to the north was mountain ranges. The world's tallest mountains that stood on the north of the Indua valley served as a guard to protect the Indus and Ganges valley from being invaded.
<u>Indus Valley is surrounded by the world's tallest mountain range of </u><u>the Hindu Kush, the Karakorum, and the Himalayas in the north.</u><u> Indus and Ganges valley form the most fertile landforms.</u>
Thus the correct answer is that the natural boundary of mountain ranges guarded Indus and Ganges Valley from being invaded.
