Find the part of the sentence that contains an error:
That the report overlooked several underlying causes (1)/ behind the public unrest were surprising (2)/ given the volume of evidence (3)/ collected over the past few months. (4)

The correct option containing the error is (2).

Explanation:
Let us analyze the structure of the given sentence:

"That the report overlooked several underlying causes behind the public unrest were surprising given the volume of evidence collected over the past few months."

1. In this sentence, the subject is the content clause (a noun clause starting with 'That'):
"That the report overlooked several underlying causes behind the public unrest"

2. A noun clause (or content clause) acting as the subject of a sentence takes a singular verb because the entire clause represents a single fact or idea.

3. Therefore, the plural auxiliary verb "were" in part (2) is incorrect. It should be replaced with the singular auxiliary verb "was" to maintain proper subject-verb agreement.

Correct Sentence:
"That the report overlooked several underlying causes behind the public unrest was surprising given the volume of evidence collected over the past few months."