Which of the following is an element
1 .carbon dioxide
2. sodium
3. ammonia
4. sand

 2)  "sodium" .

 (or, " Na " ; as it appears on the Periodic Table of Elements).

Note:

Answer choice: 1) carbon dioxide ; or, " CO₂ " ; is a compound of TWO elements.

Answer choice: 3) ammonia ; or, " NH₃ " ; is a compound of TWO or more elements; and is often found in a solution, or "mixture.

Answer choice: 4) sand — variety of types —  is a mixture of more than one elements; or most often, either: 1) a mixture of compounds; or 2) a mixture of various compounds and elements.

Answer choice: 2) sodium ; or, " Na " ; is a "single element".