(A) The given reaction involves two steps. The first step is the nucleophilic addition and the second step is the dehydration.
The hydroxide ion abstracts acidic proton to form carbonation. The carbanion attacks another carbonyl group (this is an intramolecular nucleophilic attack). This is followed by protonation of oxygen and dehydration to form the product.
(B) The given reaction is a nucleophilic substitution reaction in which chloride ion is substituted with a methyl group. The product is obtained by the rearrangement of the epoxide ring. The methyl group of the Grignard reagent attacks the terminal carbon atom of the epoxide ring to form oxide anion. The oxide anion then attacks the carbon-bearing chlorine atom.
(C) The given reaction involves two steps. The first step is the nucleophilic addition and the second step is the dehydration. The aldehyde oxygen atom is protonated. This is followed by the nucleophilic attack of the amine nitrogen on carbonyl carbon. The −OH group is then protonated and H3O+ is lost.
(D) The given reaction involves electrophilic aromatic substitution and involves dehydration (loss of a molecule of water). It also involves the formation of a carbocation intermediate.
The −OH group is protonated. This is followed by electrophilic aromatic substitution.