Keto-enol tautomerism is observed in :
(i) a.`PhCHO` , b.`PhCOCH_(3)`
c. `PhCOPh` , d. `PhCOCH_(2)COCH_(3)`

Explanation

To determine which compounds exhibit keto-enol tautomerism, we need to analyze each compound for the presence of alpha hydrogens. Tautomerism occurs when a compound can exist in two forms: a keto form (with a carbonyl group) and an enol form (with a hydroxyl group adjacent to a double bond). The presence of at least one alpha hydrogen is crucial for this process.

### Step-by-Step Solution:

1. **Analyze Compound (i): PhCHO (Benzaldehyde)**
- Structure: Benzaldehyde has the formula C6H5CHO.
- Alpha Carbon: The carbonyl carbon (C=O) is bonded to a phenyl group (C6H5) and a hydrogen atom.
- Alpha Hydrogen: There are no hydrogens on the carbon adjacent to the carbonyl group (alpha carbon).
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