Amongst the given set of reactants, the most appropriate for preparing `2^(@)` amine is..

Options

A. `2^(@)R-Br+NH_(3)`
B. `2^(@)R-Br+NaCN"followed by" H_(2)//Pt`
C. R-NH2+R-CHO gives R-NH-CH2-R
D. None of these

Explanation

To prepare a secondary amine from the given set of reactants, we need to analyze each option carefully.

### Step-by-Step Solution:

1. **Identify the Reactants**:
We have four reactants to consider. We need to determine which one can lead to the formation of a secondary amine.

2. **Examine the First Option (RBr + NH3)**:
- When RBr (an alkyl bromide) reacts with NH3 (ammonia), it will undergo nucleophilic substitution.
- The product will be RNH2 (a primary amine).
- **Conclusion**: This option yields a primary amine, not a secondary amine.

3. **Examine the Second Option (RBr + NaCN)**:
- Here, RBr reacts with NaCN (sodium cyanide).
- This reaction will produce RCN (nitrile), which upon hydrolysis and reduction (H2/Pt) will yield RCH2NH2 (a primary amine).
- **Conclusion**: This option also yields a primary amine.

4. **Examine the Third Option (RNH2 + RCHO)**:
- In this case, RNH2 (a primary amine) reacts with RCHO (an aldehyde).
- The reaction forms an imine (RNH=CR) initially.
- When this imine is reduced (using H2 in the presence of platinum), it yields RNH(CH3)R (a secondary amine).
- **Conclusion**: This option yields a secondary amine.

5. **Examine the Fourth Option**:
- Without specific details on the fourth option, we cannot analyze it further. However, based on the previous analysis, we have already identified a suitable option.

### Final Answer:
The most appropriate reactant for preparing a secondary amine is the third option: **RNH2 + RCHO**.

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