Which of the following monosaccharides are present as five-membered cyclic structure (furanose structure)?
(i) Ribose
(ii) Glucose
(iii) Fructose
(iv) Galactose
The answer is the option (i) and (iii). Due to the presence of polyhydroxyl carbonyl compound of 5 carbon atoms, ribose and fructose show the furanose structure containing five-membered cyclic structure. Thus, Option (i) and (iii) are the answers.