Introduction:
Hemiacetals are a class of organic compounds that contain both an alcohol functional group (-OH) and an aldehyde or ketone functional group (>C=O). They are formed when a carbonyl compound reacts with an alcohol. In this question, we are asked to identify an example of a hemiacetal.

Explanation:
Among the given options, the cyclic structure of glucose (option B) is an example of a hemiacetal. Glucose is a monosaccharide, which means it is a simple sugar. It exists in both linear and cyclic forms due to the presence of hemiacetal formation.

Linear and Cyclic Forms of Glucose:
Glucose can exist in a linear form, which is an open-chain structure. In this form, the carbonyl group (C=O) is present at one end of the molecule. However, in aqueous solutions, glucose predominantly exists in its cyclic form due to the reaction between the carbonyl group and the hydroxyl group on the same molecule.

Hemiacetal Formation:
In the cyclic form of glucose, the carbonyl group reacts with the hydroxyl group, resulting in the formation of a hemiacetal. Specifically, the aldehyde group on the glucose molecule reacts with the hydroxyl group on the fifth carbon atom, forming a cyclic hemiacetal. This reaction involves the formation of a new carbon-oxygen bond, converting the carbonyl group into a hemiacetal functional group.

Importance of Hemiacetal Formation in Glucose:
The formation of the hemiacetal in glucose is crucial for its stability and reactivity. It allows glucose to form glycosidic bonds with other molecules, leading to the formation of more complex carbohydrates such as disaccharides and polysaccharides.

Conclusion:
In summary, the cyclic structure of glucose (option B) is an example of a hemiacetal. In this form, the carbonyl group reacts with a hydroxyl group on the same molecule, resulting in the formation of a cyclic hemiacetal. This hemiacetal formation is important for the stability and reactivity of glucose, allowing it to participate in various biological processes.