The children left (1)/ the playground (2)/ one after one. (3)/ No error (4)

Answer

3

Explanation

oneby one oroneafteranother/the otheris the rightusage.
one by one: first one, then the next, then the next etc., separately
one after another : used for saying that actions are done / things happen with very little time between the them :
one after the other.