Monosaccharides are polyhydric aldehydes and ketones which cannot be hydrolysed into simpler carbohydrates. The monosaccharides containing `-CHO` group are called aldoses while those containing `C=O` group are called ketoses. The aldehyde group is always present at `C_(1)` while keto group is generally present at `C_(2)`. All monosaccharides are oxidised by Tollen's reagent and Fehling solution and are called reducing sugars.
The monosaccharide molecules may be assigned D and L-configurations depending upon whether the configuration of the molecule is related to D- or L-glyceraldehyde. If the `-OH` group is attached to the carbon adjacent to the `-CH_(2)OH` group (last chiral carbon) is on the right hand side, it is assigned D-configuration. The molecule is assigned L-configuration if the -OH group attached to the carbon adjacent to the `-CH_(2)OH` group is on the left. The monosaccharides contain one or more chiral carbon atoms. Pentoses and hexoses have cyclic structures furanose (five membered) and pyranose (six membered). During cyclization, `C_(1)` in aldohexoses and `C_(2)` in fructose become chiral and the newly formed -OH group may be either on the left or on the right in Fischer projection formulae. These monosaccharides, therefore, exist in two stereoisomeric forms called `alpha`-anomer and `beta`-anomer while `C_(1) and C_(2)` are called glycosidic or anomeric carbon. The bonds joining glycosidic carbon are called glycosidic linkages. `D(+)` glucose exists in two stereoisomeric forms, `alpha`-D- glucose and `beta`-D-glucose. When either of these two forms of glucose i.e., `alpha-D-`glucose are dissolved in water and allowed to stand, these get slowly converted into other form and an equilibrium mixture of both is formed. This process is called mutarotation.
Which of the following pairs give positive Tollen's test?

Options

A. Glucose, sucrose
B. Glucose, fructose
C. Hexanal, acetophenone
D. Fructose, sucrose.

Answer

To determine which pairs give a positive Tollen's test, we need to understand the conditions under which Tollen's reagent reacts with certain carbohydrates. Tollen's reagent reacts with free aldehyde groups, and some ketones can also react under specific conditions, but generally, ketones do not give a positive response.

### Step-by-Step Solution:

1. **Identify the Functional Groups**:
- Tollen's reagent specifically reacts with aldehydes. Therefore, we need to check which of the given monosaccharides or compounds contain a free aldehyde group.
- Aldoses (like glucose) contain an aldehyde group, while ketoses (like fructose) contain a ketone group.

2. **Examine Each Compound**:
- **Glucose**: This is an aldose and contains a free aldehyde group. Therefore, glucose will give a positive Tollen's test.
- **Fructose**: Although fructose is a ketose, it can exist in a cyclic form (furanose) where it has a free hemiacetal linkage. This allows fructose to also give a positive Tollen's test.
- **Sucrose**: Sucrose is a disaccharide and does not have a free aldehyde or hemiacetal group, so it will not give a positive Tollen's test.
- **Hexanol**: This is an alcohol with a free aldehyde group, so it will give a positive Tollen's test.
- **Acetophenone**: This is a ketone and does not have a free aldehyde group, so it will not give a positive Tollen's test.

3. **Conclusion**:
- The pairs that give a positive Tollen's test are those that contain at least one free aldehyde group.
- From the analysis, glucose and fructose are the only pair that gives a positive Tollen's test.

### Final Answer:
The pair that gives a positive Tollen's test is **Glucose and Fructose**.