The compound that readily tautomerizes is

Options

A. `CH_3 COCH_2 CO_2 C_2H_5`
B. `CH_3 COCH_2 CH_2 CH_3`
C. `CH_3 CO CH_2 CH_2 CH_3`
D. `(CH_3)_3 C COC(CH_3)_3`

Explanation

To determine which compound readily tautomerizes, we need to understand the concept of tautomerism. Tautomerism is a chemical phenomenon where a compound exists in two interconvertible forms, typically involving the migration of a hydrogen atom and a shift in a double bond. The most common example is the keto-enol tautomerism.

### Step-by-Step Solution:

1. **Identify the Compounds**:
We need to analyze the given options to identify which one has the potential for tautomerism. The options typically include compounds with carbonyl groups (C=O) and active methylene groups.

2. **Look for Active Methylene Groups**:
An active methylene group is a carbon atom adjacent to a carbonyl group that has hydrogen atoms attached. This group can easily lose a hydrogen atom to form an enol or an enolate ion, facilitating tautomerism.

3. **Analyze the First Option**:
The first compound mentioned has a carbonyl group and an active methylene group. This compound can readily undergo tautomerization because the hydrogen from the methylene group can migrate to the carbonyl oxygen, forming an enol.

4. **Analyze the Second Option**:
The second compound lacks the necessary active methylene group adjacent to the carbonyl. Therefore, it will not tautomerize readily.

5. **Analyze the Third Option**:
The third compound also does not have an active methylene group next to the carbonyl. The absence of a hydrogen atom on the carbon adjacent to the carbonyl means it cannot undergo tautomerization.

6. **Conclusion**:
Based on the analysis, the first compound is the one that readily tautomerizes due to the presence of an active methylene group.

### Final Answer:
The compound that readily tautomerizes is the first option.