Answer:
between 6 or 1 box.
Explanation:
Since, this is a probability question, there can be two optimum possibilities and the correct answer should lie between both the possibilities.
The minimum number of cereal boxes in which he can get all 6 prizes can be 1, because the cereal company has maximally added 6 different prizes in each box of cereal.
The maximum number of cereal boxes in which he can get all 6 prizes can be 6, because the cereal company has minimally added 1 prize in each box of cereal.