Answer:
The right answer to this question is option A. Cells
Step-by-step explanation:
Proteins are too a very important part on life, but the question is very clear, it asks whick are the smallest unit of life, so life is already a thing here, and the only alternative that mentions life here, and not a characteristic of a living being are cells. Cycles are too, part of the life of an organism, as well as dna molecules, but both of them don't configure a whole living being, that's why we cannot consider them as being fit for the answer.
Cells can include too all the other answers, they have dna molecules, that can be part of a very big cycle, and cells produce proteins too to survive.