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A. narrow is in positive form which means it is not compared to anything
B. narrower is in comparative form meaning that it is compared to one thing or two things are compared
C. narrowest is in superlative form meaning that more than two things are being compared

Analyzing the sentence we would determine that the speaker is comparing the road to their expectation because of the word "than" (like more than or less than)

:. Therefore the sentence should be:
The road was narrower than we expected.
So the answer is B