The correct option is B gaze
The required word can be determined keeping the word 'fixed' in mind.
Option C: 'Gaze' means 'to look steadily or look fixedly at something'.
'Steadily' means 'without change', 'fixed'. The word relates to the given sentence, as her 'gaze' was 'fixed' on the horizon.
Hence option C is correct.
Option A: 'Glance' means 'take a brief or hurried look'. This word does not match with 'fixed'.
Option B: 'Sight' is the 'power of seeing'; it refers to a state and cannot be 'fixed'.
Option D: 'Look' means 'to direct one's gaze in order to see something'.
It has a general meaning, as compared to 'gaze'. 'Look' also means 'glance' and 'observe'; this word does not comply with 'fixed'. So, it cannot form a complete sentence.
Hence options A, B and D are incorrect.