Attributed to is idiomatically correct, errors in other options marked in red, Correct Answer must be (C)
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Although Panama’s social spending per person is among the highest in Latin America, its students’ standardized test results rank near the bottom, an effect attributed because of a powerful teacher’s union and an inefficient civil service.


A. students’ standardized test results rank near the bottom, an effect attributed because of --> correct idiom: "A attributed to B"

B. standards of students and test results rank near the bottom, an effect attributed to --> "standards" and "results" are not different, they are related as "standardized test results "

C. students’ standardized test results rank near the bottom, an effect attributed to --> correct

D. students’ standardized test results that rank near the bottom, an effect attributed to --> no main verb

E. students’ standardized test results rank near the bottom, an effect attributed due to --> same as A