Methanol is toxic because

Methanol itself is relatively non-toxic, but its toxicity arises from its metabolic conversion in the liver by the enzyme alcohol dehydrogenase into methanal (formaldehyde), which is then further oxidized into formic acid. Formaldehyde and formic acid are highly toxic — formaldehyde coagulates protoplasm (denaturing proteins) and formic acid causes severe metabolic acidosis and damage to the optic nerve, often leading to blindness or death. Ethanol is not oxidized to acetic acid or CO in this context (option c and d are wrong); option (a) is incomplete because pure methanol does not coagulate cellular protoplasm directly.