Which is the hardest?

 - styrofoam packing peanuts
 - cobalt
 - linen handkerchief
Hard is a property. A hard material keeps its shape when you press on it with your finger.
Look at each picture, one at a time. Imagine pushing on the material shown in each picture.
Of the choices, the cobalt is the hardest. If you press on a bar of cobalt, it will not change shape.