Option B: 'Run down' is an idiomatic expression that means 'to reduce someone by criticizing them strongly'.
It forms a complete and meaningful sentence here. Hence option B is correct.
Option A: 'Talk down' is an incomplete phrase. 'Talk down to somebody' means 'to talk to someone as if they are less intelligent than you'.
If the phrase is incomplete, it won't form a complete sentence here.
Option C: 'Walk out' means 'to leave an event or a place because you are angry or disapprove of something'. The phrase should be followed by 'on', 'walk out on'. Without 'on' it sounds incomplete.
Option D: 'Stand out' means 'to be easily noticeable'. It would be followed by 'in': 'to stand out in a crowd'.
These expressions do not provide the appropriate, complete meaning to the given sentence.
Hence options A, C and D are incorrect.