The correct answer is: littered.
The word “littered” means to cover something with a large number of small objects, especially pieces of paper or rubbish. In this case, the park was covered with bits of paper after the children had left. This suggests that the children had been careless and had not thrown away their rubbish properly.
The other options are not as accurate. The word “cluttered” means to fill something with too many things, so that it is difficult to move around or find what you are looking for. The word “filled” means to cover or occupy all of the space in something. The word “scattered” means to spread something out over a large area.
In this case, the park was not filled with bits of paper, nor was it spread out over a large area. Instead, it was covered with bits of paper, which suggests that the children had been careless and had not thrown away their rubbish properly.