Triterpenoids are a class of chemical compounds composed of three terpene units, resulting in a structure that contains 30 carbon atoms. Each terpene unit is built from two isoprene units, which means that a triterpenoid, having three terpene units, consists of six isoprene units in total. Isoprene units are the fundamental building blocks of terpenes and terpenoids, with a basic molecular formula of C5H8.

**_ Four isoprene units would form a diterpenoid, not a triterpenoid. Diterpenoids consist of 20 carbon atoms, derived from four isoprene units. Three isoprene units form a sesquiterpenoid, which has 15 carbon atoms. This is different from triterpenoids, which require six isoprene units. Two isoprene units make up a monoterpenoid, consisting of 10 carbon atoms. This is significantly fewer than the 30 carbon atoms found in triterpenoids. Among the given options, six is the correct number of isoprene units in triterpenoids. This classification is based on the structure and carbon count of these compounds, which are essential for their biological and chemical properties.