The correct answer is: with.
The preposition “with” is used to indicate that someone or something is helping or assisting another person or thing. In this case, the speaker is asking the listener to help themselves to whatever they can use without wasting it. This means that the listener is free to take whatever they need from the speaker, as long as they do not take more than they need and do not waste anything.
The other options are not as appropriate in this context. The preposition “to” is used to indicate a goal or destination. The preposition “by” is used to indicate a means or method. The preposition “on” is used to indicate a surface or location. None of these options make sense in the context of the sentence “Help yourself . . . . . . . . whatever you can use without wasting.”
Here are some examples of how the preposition “with” can be used in other sentences:
- I need some help with this project.
- Can you help me with my homework?
- I’m always happy to help out with whatever I can.
- Thanks for helping me with that.
- I couldn’t have done it without your help.