The sentence is about linking the cause of riots to infrastructure and governance.
The most natural start is D — "the roots of the riots are related to" — because it directly introduces the main subject and sets up the need for an explanation.
Next is B — "the pattern of spatial growth in these towns as also to" — which specifies one factor related to the cause mentioned in D.
Then A — "an adequate physical and social infrastructure level" — follows logically, adding another factor that is part of the explanation.
Finally, C — "the failure of the government to ensure" — ties the sentence together by pointing to the lack of government action in providing the infrastructure mentioned earlier.
Thus, the full meaningful sequence is D → B → C → A, which matches option (b) DBCA.