Monosaccharides are simple sugars that cannot be hydrolyzed into smaller carbohydrate units. They are the building blocks of more complex carbohydrates like disaccharides and polysaccharides. The composition of monosaccharides can vary, but they generally contain a carbon backbone of three to seven carbon atoms.

Explanation:

- Monosaccharides are carbohydrates that are composed of carbon, hydrogen, and oxygen atoms. The general formula for a monosaccharide is (CH2O)n, where n can range from 3 to 7.
- Monosaccharides are classified based on the number of carbon atoms they contain. Some examples include trioses (3 carbon atoms), tetroses (4 carbon atoms), pentoses (5 carbon atoms), hexoses (6 carbon atoms), and heptoses (7 carbon atoms).
- Trioses, such as glyceraldehyde and dihydroxyacetone, have three carbon atoms. They are the simplest monosaccharides.
- Tetroses, like erythrose and threose, have four carbon atoms.
- Pentoses, such as ribose and deoxyribose, have five carbon atoms. They are important components of nucleic acids.
- Hexoses, like glucose and fructose, have six carbon atoms. Glucose is a primary source of energy for living organisms, while fructose is commonly found in fruits and honey.
- Heptoses, like sedoheptulose and mannoheptulose, have seven carbon atoms. They are less common than other monosaccharides.
- It is important to note that although monosaccharides generally contain three to seven carbon atoms, there are exceptions. For example, deoxyribose, a pentose, is an important component of DNA and RNA, but it only contains five carbon atoms.
- Overall, the correct answer is option 'A' (3 to 7), as monosaccharides typically have a carbon backbone consisting of three to seven carbon atoms.