The correct answer is option 3, AB de Villiers. He announced his retirement from international cricket in May 2018. AB de Villiers is a former South African cricketer who is widely regarded as one of the best batsmen of his generation. He had an illustrious career and represented South Africa in all three formats of the game - Test, ODI, and T20.

Option 1, Steve Smith, is incorrect. While Steve Smith was involved in a controversy in March 2018 and faced a ban from international cricket, he did not announce his retirement in May 2018.

Option 2, David Warner, is also incorrect. Like Steve Smith, David Warner was involved in the same controversy in March 2018 but did not retire in May 2018.

Option 4, Shane Watson, is incorrect. Shane Watson, an Australian cricketer, has retired from international cricket but not in May 2018. Watson retired from international cricket in 2016.

Therefore, the correct answer is option 3, AB de Villiers. He announced his retirement from international cricket in May 2018.