Our tragic experience in the recent past provides an index . . . . . . . . the state of lawlessness in this region. A. by B. in C. at D. of

The correct answer is: of.

An index is a measure or indicator of something. In this case, the tragic experience in the recent past is a measure or indicator of the state of lawlessness in the region.

The other options

• By suggests that the tragic experience in the recent past caused
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  the state of lawlessness. This is not necessarily the case.
• In suggests that the tragic experience in the recent past is located in the state of lawlessness. This is also not necessarily the case.
• At suggests that the tragic experience in the recent past is aimed at the state of lawlessness. This is also not necessarily the case.