Final answer:
The conversion of succinate to fumarate is a redox reaction, more specifically a dehydrogenation process, where succinate is oxidized to fumarate and FAD is reduced to FADH2. Therefore, the correct option is 1.
Step-by-step explanation:
The conversion of succinate to fumarate in the Krebs cycle is a redox reaction. This reaction is facilitated by the enzyme succinate dehydrogenase, which is located within the inner mitochondrial membrane. During this process, succinate undergoes dehydrogenation where two hydrogen atoms are removed, converting succinate into fumarate and reducing FAD to FADH2. This step does not involve hydrolysis, isomerization, or decarboxylation. Hence, it is a dehydrogenation reaction, which is a type of redox process.