It should be an alpha hydroxy ketone.
As it reacts with alkaline KMNO4 ,C6H5NHNH2,I2 NaOH and Fehling solution.
this substance reacts with I2 NaOH ,so it must contain methyl grp attached with CO grp. and hence it is not an aldehyde.
Now the question arises that ,ketone doesn't reacts with Fehling solution .ya it's correct, but except alpha hydroxy ketone.
So the substance is 3-Hydroxy-pentan-2-one.

Structure and Positive Tests for Substance C5H10O2

(a) Alkaline KMNO4:
Alkaline KMNO4 (potassium permanganate) is commonly used to oxidize various organic compounds. If substance C5H10O2 reacts with alkaline KMNO4 and gives a positive test, it indicates the presence of a compound with a double bond or an alcohol group.

(b) C6H5NHNH2:
C6H5NHNH2 (phenylhydrazine) is a reagent used to detect the presence of carbonyl compounds, such as aldehydes and ketones. If substance C5H10O2 reacts with phenylhydrazine and gives a positive test, it suggests the presence of an aldehyde or ketone group.

(c) I2 NaOH:
I2 NaOH (iodine in sodium hydroxide) is commonly used to test for the presence of compounds containing a methyl ketone functional group. If substance C5H10O2 reacts with I2 NaOH and gives a positive test, it indicates the presence of a methyl ketone group.

(d) Fehling solution:
Fehling solution is used to detect the presence of reducing sugars, specifically aldehyde or ketone functional groups. If substance C5H10O2 reacts with Fehling solution and gives a positive test, it suggests the presence of a reducing sugar or an aldehyde or ketone group.

Possible Structure for C5H10O2:
To determine the possible structure for substance C5H10O2, we need to consider the positive tests with the reagents mentioned above. Based on the positive tests, the substance may contain one or more of the following functional groups: double bond, alcohol, aldehyde, ketone, methyl ketone, or reducing sugar.

One possible structure that fulfills these criteria is:
Structure: 3-hydroxy-2-pentanone

Explanation:
- The compound 3-hydroxy-2-pentanone has the molecular formula C5H10O2, which matches the given substance.
- It contains a double bond (C=C), which can react with alkaline KMNO4.
- It has an alcohol group (-OH), which can react with phenylhydrazine and Fehling solution.
- It also has a methyl ketone group (a ketone attached to a methyl group), which can react with I2 NaOH.

By considering the positive tests and the molecular formula, 3-hydroxy-2-pentanone is a possible structure for substance C5H10O2. However, it is important to note that there may be other structures that can also give positive tests with the mentioned reagents, so further analysis and characterization techniques should be employed to confirm the exact structure.