For osazone formation, the effective structural unit necessary is

Options

A. `{:(CH_2OH_3),(|),(CO),(|):}`
B. `{:(CH_2OH),(|),(CO),(|):}`
C. `{:(CH_2OH),(|),(CHOCH_3),(|):}`
D. `{:(CH_2O),(|),(CHOCH_3),(|):}`

Answer

To determine the effective structural unit necessary for osazone formation, we need to analyze the options provided and understand the requirements for osazone formation.

### Step-by-Step Solution:

1. **Understanding Osazone Formation**:
- Osazone formation occurs when reducing sugars react with phenylhydrazine. The key functional groups involved are the aldehyde (CHO) or ketone (C=O) groups present in the sugar molecules.

2. **Identifying the Key Structural Units**:
- The osazone formation requires a structural unit that contains both an aldehyde or ketone group and a hydroxyl group (OH). This is because the reaction involves the carbonyl carbon of the sugar reacting with phenylhydrazine.

3. **Analyzing the Options**:
- **Option 1: CH2OCH3CO** - This structure does not contain the necessary aldehyde or ketone group for osazone formation.
- **Option 2: CH2OHCO** - This structure contains both a hydroxyl group (OH) and a carbonyl group (C=O), making it a suitable candidate for osazone formation.
- **Option 3: CH2OHCHOCH3** - This structure has a hydroxyl group, but the presence of CHO and CH3 complicates the formation of osazone as it does not provide a clear carbonyl group for reaction.
- **Option 4: CH2O and CHOCH3** - This structure is ambiguous and does not clearly indicate a suitable carbonyl group for osazone formation.

4. **Conclusion**:
- Based on the analysis, **Option 2: CH2OHCO** is the effective structural unit necessary for osazone formation, as it contains the required hydroxyl and carbonyl groups.

### Final Answer:
**The effective structural unit necessary for osazone formation is CH2OHCO (Option 2).**