The correct option is B were walking on an uneven road.
This question requires you to make an inference, i.e. choose an answer
that is not directly stated in the passage but is implied.
There is nothing in the passage to suggest that the narrator and his friend were trying to keep their movements secret, so option D is incorrect.
Though they watch the distant light, the narrator also observes other aspects of the landscape such as the craggy hills nearby. There is therefore nothing in the passage to suggest that their attention was fixed only on the distant light to the exclusion of all else. Therefore option C is incorrect.
The narrator and his friend could view the hills around them as well as the light ahead of them, so their eyesight must have been good. Option A is incorrect.
This leaves option B which indicates that they were walking on an uneven road, which they probably could not see in the darkness, causing them to stumble along slowly in the darkness.