The answer is (CH2O)n.
Monosaccharides consists of carbon, hydrogen, and oxygen in ratio 1:2:1.
The basic formula of monosaccharides is (CH2O)n. Let's take for example a glucose, which is monosaccharide. It is known that the formula of the glucose is C6H12O6, which means that it consists of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. So, in formula (CH2O)n, it must be n=6, so a glucose can be also represented as (CH2O)6.
