Here it is
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d because of mesomery during addition of Grignard.

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.
[attachment id=0 msg=3050] 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 butanal.
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