A competitive inhibitor of succinate dehydrogenase is

Explanation:
malonate( a 3 C containing dicarboxylic acid) ,on account of its structural similarity to succinate(a 4 C containing dicarboxylic acid) ,competes with it for binding to same active site,hence acting as a competitive inhibitor of succinate dehydrogenase(SDH) enzyme and thereby, decreasing the efficiency of transformation of succinate to fumarate in Krebs cycle.
Its structural similarity to succinate along with reaction intervened can be observed here:

Final Answer:
Hence,the correct answer is option D. Malonate