Organocuprate Reaction Mechanism
The reaction proceeds in two steps:
- 1,4-addition of the organocuprate: The organocuprate reagent (MeMgBr/CuI) adds to the α, β-unsaturated ketone in a 1,4-conjugate addition manner. This results in the formation of a ketone with a methyl group at the β position.
- Alkylation of the ketone: The resulting ketone reacts with n-propyl iodide (nPrI) in an alkylation reaction. The enolate of the ketone attacks the alkyl halide, adding the n-propyl group to the α-carbon.

Conclusion:
Therefore, the major product is option (3).