Find the part of the sentence that contains an error:
That the report failed to address the root causes (1)/ of the community unrest were surprising (2)/ given the exhaustive data (3)/ compiled over several months. (4)

Correct option is B

Option (b) contains an error.
Detailed explanation of error: The subject of the sentence is the entire that-clause “That the report failed to address the root causes,” which is grammatically singular.
Therefore, the verb should be was, not were: “That the report failed … was surprising.”
The remaining parts are fine: “of the community unrest” (correct prepositional phrase), “given the exhaustive data” (correct concessive/causal adjunct), and “compiled over several months” (reduced relative clause).
Grammatical rule used: A that-clause functioning as a subject takes a singular verb. Subject–verb agreement requires the finite verb to agree in number with the grammatical subject, not with nearby plural nouns.
Example: “ That so many applicants withdrew was unexpected,” not “ were unexpected.”
Information booster: When an -ing/reduced clause or a clause like what/that + clause acts as subject, default to singular verb (What he said is true; That they left early was wise), regardless of plural nouns contained inside the clause.