The correct option is A the number of carbon atoms
Monosaccharides are the simple sugars that cannot be hydrolysed further to give simpler sugars. On the basis of number of carbon atoms, monosaccharides can be classified as triose (with 3 carbon atoms), tetrose (with 4 carbon atoms), pentose (with 5 carbon atoms), hexose (with 6 carbon atoms), septose (with 7 carbon atoms) and so on.