若画线的是play badminton,则应该这样提问：
What do they often do in the afternoon?
若画线的是often,则应该这样提问：
How often do they play badminton in the afternoon?
若画线的是in the afternoon,则应该这样提问：
When do they often do in the afternoon?
若画线的是They,则应该这样提问：
Who often plays badminton in the afternoon?
因为最后“who"相当于是第三人称单数,所以"plays"要加S.