Which of the following is NOT a type of isomer?

The correct answer is based on the fact that isomers are variations of compounds that demonstrate different structural or spatial arrangements while having the same molecular formula. Linkage isomers, ionization isomers, and hydration isomers are all recognized types of isomers.

Linkage isomers arise when a ligand can attach to a metal center in different ways, resulting in different structural configurations of the complex. Ionization isomers occur when the composition of the ions in solution changes, leading to different ionic forms of coordination compounds. Hydration isomers differ by the number of water molecules coordinated to the metal center versus those in the outer sphere of the complex.

In contrast, "isothermal isomers" is not a recognized term in the study of isomerism. The term does not correspond to a specific type of isomerization and does not fit within the established classifications of isomers. Hence, this choice is correctly identified as NOT a type of isomer.