Dehydrogenation of succinic acid to fumaric acid requires the following hydrogen carrier:

Succinate is dehydroge nated to fumarate, an unsaturated dicarboxylic acid, by succinate dehydrogenase. The hydrogen atoms are accepted by FAD. The FADH2 then enters into ETC to generate ATPs. The enzyme is a flavoprotein. Reference: Vasudevan 7th  ed, pg 248