Answer:

To determine which of the given options will produce propanenitrile as the major product, we need to consider the reaction between the alkyl halide and the nucleophile (cyanide ion) in each case.

1. Ethyl bromide + alcoholic KCN
2. Propyl bromide + alcoholic KCN
3. Propyl bromide + alcoholic AgCN
4. Ethyl bromide + alcoholic AgCN

Reaction Mechanism:
In the presence of a strong nucleophile like cyanide ion, the reaction proceeds through an SN2 (substitution nucleophilic bimolecular) mechanism. This means that the nucleophile attacks the alkyl halide molecule from the backside, resulting in the displacement of the leaving group (halide ion). The reaction follows a one-step concerted process.

Factors Affecting SN2 Reactions:
SN2 reactions are influenced by several factors including the nature of the alkyl halide, the strength of the nucleophile, and the solvent used.

1. Nature of the Alkyl Halide:
The reactivity of alkyl halides towards SN2 reactions follows the order: methyl > primary > secondary > tertiary. This is because steric hindrance increases with the number of alkyl groups attached to the carbon atom bearing the leaving group. In the given options, both ethyl bromide and propyl bromide are primary alkyl halides.

2. Strength of the Nucleophile:
Strong nucleophiles favor SN2 reactions. In this case, both KCN and AgCN are strong nucleophiles capable of participating in SN2 reactions.

3. Solvent:
The solvent used can affect the reaction rate and product distribution. In this case, an alcoholic solvent (alcohol) is used in all options.

Analysis of Options:
a) Ethyl bromide + alcoholic KCN:
- Ethyl bromide is a primary alkyl halide.
- KCN is a strong nucleophile capable of participating in SN2 reactions.
- Alcoholic solvent is used.
- The reaction will proceed via an SN2 mechanism, resulting in the formation of propanenitrile as the major product.

b) Propyl bromide + alcoholic KCN:
- Propyl bromide is also a primary alkyl halide.
- KCN is a strong nucleophile capable of participating in SN2 reactions.
- Alcoholic solvent is used.
- The reaction will proceed via an SN2 mechanism, but the product formed will be butyronitrile, not propanenitrile.

c) Propyl bromide + alcoholic AgCN:
- Propyl bromide is a primary alkyl halide.
- AgCN is a strong nucleophile capable of participating in SN2 reactions.
- Alcoholic solvent is used.
- The reaction will proceed via an SN2 mechanism, but the product formed will be butyronitrile, not propanenitrile.

d) Ethyl bromide + alcoholic AgCN:
- Ethyl bromide is a primary alkyl halide.
- AgCN is a strong nucleophile capable of participating in SN2 reactions.
- Alcoholic solvent is used.
- The reaction will proceed via an SN2 mechanism, but the product formed will be ethanenitrile, not propanenitrile.

Conclusion:
From the analysis above, it