The city was plunged . . . . . . . . darkness due to a sudden power failure.

The correct answer is C. The city was plunged into darkness due to a sudden power failure.

The other options are incorrect because they

do not accurately describe the situation. Option A, “through,” suggests that the city was somehow passed through darkness. Option B, “to,” suggests that the city was transported to a dark place. Option D, “under,” suggests that the city was buried under darkness.

The word “plunged” means to be suddenly and deeply immersed in something. In this case, the city was suddenly and deeply immersed in darkness. This is best described by the word “into.”