Monosaccharides contain the elements carbon, hydrogen and oxygen, with twice as many H to O atoms. An example of a monosaccharide is glucose. Monosaccharides are the building blocks of more complex carbohydrates.
Two monosaccharides can join by a covalent bond called a glycosidic linkage to form a disaccharide. An example is sucrose, or table sugar which is formed when the monosaccharides glucose and fructose are chemically joined. When several monosaccharides join by glycosidic linkages, this forms large polymers called polysaccharides including starch and cellulose.
A diagram depicting a monosaccharide can show its carbon skeleton in a linear arrangement. However, in an aqueous solution, the carbon skeleton of sugar can form a ring.
The answer to your question is that the ring structure of glucose indicates that it is a monosaccharide or choice A. If it were a disaccharide, there would be two rings therefore it cannot be answer B. It cannot be choice C as a fatty acid is a long linear skeleton of carbon with mainly Hydrogen atoms attached. The nucleotide in choice D contains either the sugar ribose or deoxyribose attached to a phosphate group and to a nitrogenous base. It is more complex than just a single ring structure. For choice E, amino acids are building blocks of protein and are not found in a ring formation.
I have included a diagram of glucose to illustrate why choice A is the correct response.