Lack of enough blood

Answer: Option C

One word substitute is Anaemia.

Amnesia: a partial or total loss of memory.
Insomnia: habitual sleeplessness; inability to sleep.
Anaemia: a condition in which there is a deficiency of red cells or of haemoglobin in the blood, resulting in pallor and weariness.
Allergy: a damaging immune response by the body to a substance, especially a particular food, pollen, fur, or dust, to which it has become hypersensitive.