The answer is A, CO.
Out of all 4 choices, only A is not an element, but a compound. Compound are made out of different elements while element itself cannot be further separated no matter what methods we use. In A, CO, is carbon monoxide. It contains carbon and oxygen these 2 elements. Both carbon and oxygen cannot be further separated, but carbon monoxide can be separated back to carbon and oxygen.
On the other hand, B,C, and D, are symbols of elements helium, sodium, and sulphur respectively. They cannot be further separated into other smaller substances thus they're elements.
