The tricarboxylic acid (TCA) cycle, also known as the citric acid cycle or Krebs cycle, involves the oxidative decarboxylation of malate to oxaloacetate and of isocitrate to alpha-ketoglutarate. However, the latter reaction is followed by another decarboxylation when alpha-ketoglutarate is converted to succinyl-CoA. So in total, there are two decarboxylation reactions per TCA cycle.

So, the correct answer is Option D : Twice.