Unlike the buildings in Mesopotamian cities, which were arranged haphazardly, the same basic plan was followed for all cities of the Indus Valley: with houses laid out on a north-south, east-west grid, and houses and walls were built of standard-size bricks.
A) the buildings in Mesopotamian cities, which were arranged haphazardly, the same basic plan was followed for all cities of the Indus Valley: with houses - Unlike tells us that this sentence presents a contrast. What follows Unlike is the buildings in Mesopotamian cities. Now, these buildings have been illogically contrasted with the same basic plan that is subject of the main clause ; usage of which -- not clear what does which refer to ; structural error -- colon used to join independent clauses
B) the buildings in Mesopotamian cities, which were haphazard in arrangement, the same basic plan was used in all cities of the Indus Valley: houses were - same as A
C) the arrangement of buildings in Mesopotamian cities, which were haphazard, the cities of the Indus Valley all followed the same basic plan: houses - This option illogically presents the contrast between the arrangement of the buildings and the cities of the Indus Valley. These two entities are not logically parallel ; structural error
D) Mesopotamian cities, in which buildings were arranged haphazardly, the cities of the Indus Valley all followed the same basic plan: houses were - Correct
E) Mesopotamian cities, which had buildings that were arranged haphazardly, the same basic plan was used for all cities in the Indus Valley: houses that were - This sentence illogically presents the contrast between Mesopotamian cities and the same basic plan.
This choice repeats the structural error of Choice C. The noun after the colon houses does not have a verb.
Answer D
.