The correct answer is C. Shredding.
Slashing is a violent act of cutting something with a sharp object. Ripping is the act of tearing something apart with force. Disposing is the act of getting rid of something. Shredding is the act of cutting something into small pieces, usually with a machine.
In the context of the question, the most likely action is shredding, as this is the most efficient way to cut waste paper into pieces. Slashing and ripping would be more time-consuming and would not produce as small of pieces. Disposing would not be the most efficient way to cut waste paper into pieces, as it would require more time and effort.