What substance can not be broken down? mixture,element,or compound

Well really you can break down any of them.

But this seems to be chemistry question so the answer will be element. Here's why:

A compound is a chemical formed either by covalent or ionic bonding (keeping it simple for now). You can break a compound down into its constituent elements via a variety of chemical processes.

A mixture of a mix of different chemicals that have not reacted chemically. Coffee (drink) is a mixture, not a compound whereas salt is a compound. You could get the constituent parts of coffee by, say, filtration or evaporation etc but this won't work for a compound or element.

An element cannot be broken by chemical means. You can break down elements e.g. nuclear fission 'breaks' a atomic nucleus into other elements. But it cannot be done by simple chemical means.

It is all a touch more complex than that but that's the general gist.

EDIT Think element = elementary i.e. the simplest form. Compound is to add together (e.g. to compound a problem) and mixture is when you mix two or more things together.

Umm, how technical an answer are you looking for?

i dont know but there's definatley some shit on the inside of my mates toilet than cant be broken down!

elements (guessing)

any of those can be 'broken down' into a lot of parts.. so broken down how?