Final answer:
In step 9 of the citric acid cycle, the conversion of fumaric acid to malic acid is classified as isomerization.
Step-by-step explanation:
The conversion of fumaric acid to malic acid in step 9 of the citric acid cycle is classified as isomerization. Isomerization is a process where a compound undergoes a rearrangement of its atoms, resulting in a different structural isomer. In this step, fumaric acid is transformed into malic acid by rearranging the position of the double bond.