The correct option is B Sight
Option B is the correct answer because 'sight' means to see or observe something. It is mentioned in the sentence that the person's attention is fixed on the horizon hence 'sight' is the right answer conveying the same.
Option A is incorrect because glance means to look at something quickly. It is mentioned in the sentence that the person's attention is fixed hence glance is incorrect.
Option C is incorrect because gaze means looking at something steadily out of admiration. respect, etc. In this context 'gaze' isn't the appropriate word as the word 'fixed' doesn't convey any feeling of admiration.
Option D is grammatically incorrect in this context.