Step 1 - Understanding what is a pure element
A pure substance is a substance that is not mixed up with any other substances. Water, for example, is a pure substance, as well as alcohol (ethanol). But if we mixture water and alcohol, we have a mixture, not a pure substance.
A pure substance is therefore composed only of molecules of the same kind. However, a pure substance is not the same thing as a pure element. A substance may be formed by different elements, such as water, which is formed by two elements, H (hydrogen) and O (oxygen).
A pure element is thus a pure substance formed by only one type of element. Some examples are hydrogen gas, H2, oxygen gas, O2, and so on. Note that they are pure elements because they are pure substances (they're not mixed with anything else), but are formed by just one kind of atom.
Step 2 - Using the definition of pure element to solve the question
We are looking for a substance formed by only one kind of atoms. Therefore, in the pictures, we should only select those squares containing balls (atoms) of the same kind.
Note that the 2nd and the 3rd squares both have several kinds of atoms, thus constituting a mixture and a pure substance, respectively, but never a pure element.
Squares 1 and 4, on the other hand, show only one kind of atom. They correctly represent, therefore, pure elements.