Which one of the following is a non-reducing sugar?

Sucrose

The correct answer is option 2. sucrose.

To determine which of the given sugars is a non-reducing sugar, let us briefly review what constitutes a reducing sugar and then analyze each option: 

Reducing sugars are those that have a free aldehyde group or a free ketone group that can be oxidized. In the presence of an oxidizing agent, reducing sugars can reduce it, thus being "reducing sugars." Common tests for reducing sugars include Benedict's test and Fehling's test, where a positive result indicates the presence of a reducing sugar.

Analysis of Each Option:

1. Glucose:

Type: Aldohexose (contains an aldehyde group).

Reducing or Non-reducing: Reducing sugar because it has a free aldehyde group.

2. Sucrose:

Type: Disaccharide (composed of glucose and fructose linked by a glycosidic bond).

Reducing or Non-reducing: Non-reducing sugar because the glycosidic bond involves the anomeric carbon of both glucose and fructose, rendering them unable to open into a free aldehyde or ketone form.

3. Fructose:

Type: Ketohexose (contains a ketone group).

Reducing or Non-reducing: Reducing sugar because it has a free ketone group that can participate in redox reactions.

4. Arabinose:

Type: Aldopentose (contains an aldehyde group).

Reducing or Non-reducing: Reducing sugar because it has a free aldehyde group.

Conclusion: The non-reducing sugar among the options provided is 3. Sucrose.

This is because sucrose has a glycosidic bond between the anomeric carbons of glucose and fructose, preventing it from having a free aldehyde or ketone group necessary for it to act as a reducing sugar.