The correct option is A notice
The appropriate word that fills the blank must be a verb (considering the normal word order of a sentence: subject (you) + verb).
From the options, 'notice' means 'to become aware of', when placed in the sentence it means 'if you become aware of something'; hence it is appropriate in meaning and option A is correct.
'To view' means 'to look at or inspect', when placed here: 'view something amiss' does not sound right, as 'view' does not go well with 'amiss'. Hence B is incorrect.
'Sight' means 'to catch an initial glimpse of', it's a very specific nature of glimpse described, whereas the sentence calls for a simpler word, 'notice'. Hence C is incorrect.
'Glimpse' means 'a momentary or partial view'; also, 'glimpse' is a noun, its verb form 'glimpsed' isn't in the option. Hence option D is incorrect.