Acetaldehyde with few drops of concentrated sulphuric acid
You're not wrong.

Your mechanism describes the classic acid catalyzed synthesis of 3-hydroxybutanal by dimerisation of acetaldehyde (Borodin 1869, Wurtz 1872).


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However, under different conditions acetaldehyde forms different addion and condensation products. E.g. with concentrated sulphuric acid at low temperatures metaldehyde is formed whereas at room temperature paraldehyde (the answer given in your book) is formed.