The grammatically incorrect sentences are:B, C and D only

B. I have met his friend yesterday. (grammatically incorrect) This sentence mixes present perfect (“have met”) with a past adverb (“yesterday”). The correct construction would be either “I met his friend yesterday” (simple past) or “I have just met his friend” (present perfect with recent time implication).

C. She went to Dehradun day before yesterday. (This sentence is incorrect due to the missing article "the" before "day before yesterday.")

D. He has bought a house three years ago. (grammatically incorrect) Similar to B, this sentence mixes present perfect and past adverb. The correct options would be “He bought a house three years ago” (simple past) or “He has just bought a house” (present perfect with recent time implication).

Note: As per the NTA, the correct answer was Option 4 i.e. B and D only.