Atherosclerosis and embolism are both caused by a blockage of the blood vessel, so it's unlikely to be either of those.
Arteriosclerosis is to do with the thickening of the arteries, whereas a vascular spasm is the tightening of a blood vessel.
Since vascular occlusion relates to the blockage of a blood vessel, it should either be arteriosclerosis or vascular spasm, however, I can not say which one. Since arteriosclerosis is the thickening of the arteries over time and a spasm is the partial/complete blockage of the vessel and is also spontaneous, I would personally think the answer is arteriosclerosis, since it doesn't involve blockage, but rather, a gradual reduction in diameter.