Which of the following groups shows the strongest` -R` effect?

Options

A. `-COOR`
B. `-CN`
C. `-NO_(2)`
D. `-CHO`

Explanation

To determine which group shows the strongest -R (minus resonance) effect, we need to analyze the groups based on their ability to withdraw electrons through resonance. The -R effect is typically exhibited by groups that can stabilize a negative charge or pull electron density away from the rest of the molecule.

### Step-by-Step Solution:

1. **Understanding the Resonance Effect**:
- The resonance effect can be categorized into two types: +R (positive resonance) and -R (negative resonance).
- The -R effect indicates that a substituent withdraws electron density from the rest of the molecule through resonance.

2. **Identifying Electron-Withdrawing Groups**:
- Groups that exhibit a -R effect are generally electron-withdrawing groups (EWGs).
- Common examples of -R groups include nitro (NO2), cyano (CN), carbonyl (C=O), and sulfonyl (SO3).

3. **Comparing the Strength of -R Groups**:
- The strength of the -R effect can be compared based on the electronegativity of the atoms involved and their ability to stabilize negative charge through resonance.
- The order of -R effect strength is generally:
- NO2 > CN > COOH > CHO > SO3 > CH3.

4. **Identifying the Strongest -R Group**:
- Among the groups mentioned, the nitro group (NO2) is known to be the strongest -R group due to its high electronegativity and ability to stabilize negative charge effectively through resonance.

5. **Conclusion**:
- Therefore, the group that shows the strongest -R effect is the nitro group (NO2).

### Final Answer:
The group that shows the strongest -R effect is **NO2 (nitro group)**.