We have to consider that both red and white blood cells are cells that in fact are present in the blood, and perform different functions, such as oxygen transport, and the defense against pathogenic agents, like bacteria or viruses.
Platelets are also blood cells, but their function is to avoid the blood to coagulate itself when, for example, we have a wound in our skin.
On the other hand, the structure which is not part of our blood is the arteries, since this is a channel where it flows along our whole bodies.
The correct answer then is the last one: D. arteries.