A competitive inhibitor of succinic dehydrogenase is:

Explanation

Malonate is a competitive inhibitor of succinic dehydrogenase. It mimics the structure of succinate, the natural substrate, allowing it to bind to the enzyme's active site and block succinate from binding. This inhibition can slow down the Krebs cycle. The other options, alpha-ketoglutarate, malate, and oxaloacetate, are substrates or intermediates in the Krebs cycle but do not inhibit the activity of succinic dehydrogenase.