In TCA cycle, isomerisation of citrate is followed by how many decarboxylation(s) to form succinic acid ?

The correct option is D Two

A several intermediate compounds are formed, which contain three carboxylic groups and therefore the process is called as tricarboxylic acid cycle (TCA).

First, the condensation of acetyl group with oxaloacetic acid (OAA) and water takes place to yield citric acid, catalysed by the enzyme citrate synthase and a molecule of CoA is released. Citrate is then isomerised to isocitrate, which is followed by two successive steps of decarboxylation, leading to the formation of α-ketoglutaric acid and then succinyl-CoA.

So, the correct answer is "two"