In this reaction
nucleophilic addition followed by protonation can take place in two ways.This is due to resonance between the conjugated Pi bond and carbon double bond with the oxygen.
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The additions which can take place are classified -as 1,4 addition and 1,2 addition.
In
1,4 addition the nucleophile of the Grignard reagent attacks on the alkene carbon which is beta to carbonyl group and the double bond shifts between carbonyl carbon and Alpha carbon to the carbonyl carbon atom.
In our question 1,4 addition results in the formation of
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