This question tests knowledge on carboxylic acid derivatives and the formation of the carboxylic acid derivatives. , or thionyl chloride, reacts with a carboxylic acid to form an acid chloride. The choice that replaces the with a . The reaction also expels .

5-hydroxyvaleric acid (5-hydroxypentanoic acid) is a five-carbon carboxylic acid alcohol and will produce the required lactone. This reaction is typically acid-catalyzed. Pentanedioic acid (glutaric acid) is a linear dicarboxylic acid, 4-hydroxybutyric acid only has four carbons and will not produce the desired product without being modified.

In the HVZ reaction of a carboxylic acid, a bromine is added to the alpha carbon. Phosphorus catalyzes the reaction and allows for the formation of an acyl halide. Acyl halides readily undergo enol-ketone tautomerization. The enol form uses electrons from the carbon-carbon double bond to bond to the bromine. In water, the two bromine molecules convert back to a carboxylic acid with one bromine on the alpha carbon.