Answer is A (Acetylcholine):The most impoant biochemical abnormality in Alzheimer's disease is the decrease in coical levels of AcetylcholineBiochemically Alzheimer's disease is associated with a decrease in the coical levels of several proteins and neurotransmitters especially acetylcholine, its synthetic enzyme choline-acetyl-transferase, and nicotinic cholinergic receptors. Reduction of acetylcholine is related in pa to degeneration of cholinergic neurons in the nucleus basalis of Meyne (NBM) that projects through the coex. There is also noradrenergic and serotonergic depletion due to degeneration of brainstem nuclei such as the locus ceruleus and dorsal raphe' - Harrison's