Which of these is a compound?

salt water

salad

carbon dioxide

or potassium

The correct answer is carbon dioxide.

Carbon dioxide is contained of carbon and oxygen atoms which are chemically combined together. They cannot be separated by just using simple physical methods such as decantation or evaporation. They can only separate into carbon and oxygen using chemical method such as electrolysis. These are some properties of what compound has.

In opposite, salt water, is a mixture. It can easily be separated back into salt and water by using methods like evaporation or crystallization.

Salad is also a mixture, it is mixed of different food that are made from different molecules such as cabbage, sauce, apples etc.

And Potassium is an element. It cannot be further separated into anything simpler. It is not a compund.

So, the only correct answer here is carbon dioxide.