A substance `C_(5)H_(10)O` does not reduce Fehling's solution, forms a phenylhdrazone, shows the haloform reaction, and can be converted into n-pentane by strong reduction. The structure formula of the original substance is:

Options

A. `CH_(3)COC_(3)H_(7)`
B. `C_(2)H_(5)COC_(2)H_(5)`
C. `CH_(3)COCH(CH_(3))_(2)`
D. `C_(4)H_(9)CHO`

Explanation

To solve the problem, we need to analyze the information given about the substance with the molecular formula C5H10O. We will go through the clues step by step to determine the correct structure.

### Step 1: Analyze the Molecular Formula
The molecular formula is C5H10O. This indicates that the compound is likely a ketone or an aldehyde, as it contains a carbonyl group (C=O).

**Hint 1:** Look for functional groups that fit the molecular formula C5H10O.

### Step 2: Fehling's Solution Test
The substance does not reduce Fehling's solution. This indicates that the compound does not have an aldehyde group, as aldehydes typically reduce Fehling's solution.

**Hint 2:** Remember that the inability to reduce Fehling's solution suggests the absence of an aldehyde.

### Step 3: Formation of Phenylhydrazone
The compound forms a phenylhydrazone, which indicates that it contains a carbonyl group (C=O). This means the compound is likely a ketone since we have already ruled out aldehydes.

**Hint 3:** The formation of phenylhydrazone confirms the presence of a carbonyl group.

### Step 4: Haloform Reaction
The substance shows a positive haloform reaction. This reaction is characteristic of methyl ketones (compounds with the structure RCOCH3). Therefore, the compound must contain a methyl ketone functional group.

**Hint 4:** Identify the structure of methyl ketones, which have the general formula RCOCH3.

### Step 5: Strong Reduction to n-Pentane
The compound can be converted into n-pentane by strong reduction. This means that the original compound must have a structure that contains all five carbon atoms in a chain, with the carbonyl group located in such a way that it can be fully reduced to a saturated hydrocarbon.

**Hint 5:** Consider the structure of the compound and how it can be reduced to a straight-chain alkane.

### Step 6: Determine the Structure
Based on the above clues, we can deduce that the compound must be a methyl ketone with a total of five carbon atoms. The only structure that fits all these criteria is:

- **Structure:** CH3CO-CH2-CH2-CH3 (2-pentanone)

This structure has:
- A carbonyl group (C=O) at the second carbon (making it a ketone).
- It does not reduce Fehling's solution (no aldehyde).
- It can form a phenylhydrazone.
- It shows a positive haloform reaction.
- It can be reduced to n-pentane (C5H12).

### Conclusion
The structure formula of the original substance is CH3CO-CH2-CH2-CH3, which corresponds to option 1.

**Final Answer:** The structure formula of the original substance is CH3CO-CH2-CH2-CH3 (2-pentanone).