The compound which reacts with a base in the soap-making process is a/an ____________.

Saponification is simply the process of soap making. Soaps are nothing just potassium or sodium salts of long-chain fatty acids. An ester reacts with an inorganic base to give soap and alcohol during the process of saponification. In other words, it is the hydrolysis of an ester with NaOH or KOH to give sodium or potassium salt of the acid and alcohol. Animal fats and vegetable oil are triglycerides. When these animal fats and vegetable oil are reacted with sodium hydroxide, then hard soap is created. When these animal fats and vegetable oil are reacted with potassium hydroxide, then soft soap is created. 

$\mathrm{Ester} + \mathrm{Base} \stackrel{ }{\to } \mathrm{Alcohol} + \mathrm{Soap}$ (Saponification)

Hence, The compound which reacts with a base in the soap-making process is an Ester.