【正确答案】 B

【答案解析】Logical predication; Rhetorical construction
It is difficult to tell which parts of this sentence go together because of errors and confusion in the underlined portion. Living allowances is the counterpart of paychecks, so it is better to say governments... substitute living allowances for the paychecks of their employees because it makes the substitution clearer. This change also makes it easier to correct the modification error that appears in the phrase assigned by them, which incorrectly modifies paychecks rather than employees. The modifying clause who have been assigned clearly describes employees and fits into the remaining part of the sentence, to the United Nations.
A Assigned by them incorrectly and illogically modifies paychecks.
B Correct. In this sentence, the meaning is clearer, because paychecks is separated from employees. The relative clause clearly modifies employees.
C Having been assigned illogically modifies governments.
D The correct construction is substitutes x for y, not substitutes x in place of y. The construction following paychecks is wordy and awkward.
E The correct construction is substitutes x for y, not substitutes x in place of y. The construction following employees is wordy and awkward.
The correct answer is B.