Mildest Reducing Agent for Carbonyl Group Reduction

To determine the mildest reducing agent that selectively reduces only the carbonyl group in the presence of other functional groups like nitro, carboxyl, double bond, and ester groups, we need to consider the reactivity and selectivity of various reducing agents.

The options given are:
a) LiAlH4
b) Na-NH3
c) NaBH4
d) H2-Ni

LiAlH4 (Lithium Aluminum Hydride)
LiAlH4 is a powerful reducing agent that can reduce a wide range of functional groups, including carbonyl, nitro, carboxyl, double bond, and ester groups. It is highly reactive and can cause uncontrolled reactions, making it unsuitable for selective reduction of the carbonyl group in the presence of other functional groups. Therefore, option a) can be eliminated.

Na-NH3 (Sodium in Liquid Ammonia)
Na-NH3 is also a strong reducing agent, but it is less reactive compared to LiAlH4. It can selectively reduce carbonyl groups, but it can also reduce other functional groups under certain conditions. Therefore, option b) is not the mildest reducing agent for selective carbonyl group reduction.

NaBH4 (Sodium Borohydride)
NaBH4 is a milder reducing agent compared to both LiAlH4 and Na-NH3. It is primarily used for the selective reduction of carbonyl groups while leaving other functional groups unaffected. NaBH4 is generally not reactive towards nitro, carboxyl, double bond, and ester groups, making it the ideal choice for the given scenario. Therefore, option c) is the correct answer.

H2-Ni (Hydrogen with Nickel Catalyst)
H2-Ni is not a reducing agent itself but a hydrogenation catalyst. It is typically used for the reduction of double bonds or triple bonds to form single bonds. It is not suitable for the selective reduction of carbonyl groups in the presence of other functional groups. Therefore, option d) can be eliminated.

Conclusion
Among the given options, NaBH4 is the mildest reducing agent that selectively reduces only the carbonyl group while leaving other functional groups like nitro, carboxyl, double bond, and ester groups unaffected.