A study concludes that the Indus civilization, which existed 6,000 years ago, experienced violence and disease of such severity
they eroded the ability of the population to stay in their cities and, the result is, to remain intact as a civilization.
A. they eroded the ability of the population to stay in their cities and, the result is, to remain - will require 'that' in the beginning.
B.
that they eroded the ability of the population
to stay in their cities and, as a result,
to remain -usage of 'that' is correct and the sentence is parallel
C. that they eroded the ability of the population to stay in their cities and, the result of this,
they were unable to remain -'they' is referring to what? Ambiguous.
D. that they eroded the ability of the population to stay in their cities, and resulted
in being unable to remain -wrong construction and weird usage in the sentence.
E.
as to erode the ability of the population for staying in their cities,
resulting in being unable to remain -In the first part, the sentence would require 'that' and in the second part, the construction is wrong.
IMO answer is Option B