To identify compound X ($C_5H_{10}O$):

1. Phenylhydrazone Formation: The reaction with phenylhydrazine indicates the presence of a carbonyl group ($>C=O$). This confirms X is an aldehyde or ketone, eliminating the alcohol (Option D).
2. Negative Tollen's Test: Tollen's reagent oxidizes aldehydes to carboxylic acids (silver mirror). A negative result indicates X is not an aldehyde, eliminating Pentanal (Option A). X must be a ketone.
3. Negative Iodoform Test: The iodoform test detects the presence of a methyl ketone group ($CH_3-C=O$).

• 2-Pentanone ($CH_3-CO-CH_2CH_2CH_3$) contains this group and would give a positive test.
• 3-Pentanone ($CH_3CH_2-CO-CH_2CH_3$) does not contain a methyl group attached directly to the carbonyl carbon. It gives a negative test.

4. Reduction: Reduction of 3-pentanone yields n-pentane, consistent with the problem statement.