All terpenes have carbon skeletons made up of -------
A. isoprene units
B. vinyl units
C. alkenes
D. Ethylene units

Hint: To know the correct option of the given question, we should have knowledge about what are terpenes. The terpenes are the units which contain alternate double bonds in its structure. It is present as a chief component in natural rubber.

Complete answer:
To get the appropriate answer, first of all let us know, what are terpenes.
Terpenes is an organic compound that is usually produced from plants and some insects too. They are the major component of resin. We see terpenes mainly in essential oils (which are derived from many plants and flowers). As we know, essential oils are used as fragrance in perfumes, additives and flavouring in food items.
Terpene is basically a hydrocarbon and the whole carbon skeleton is made up of isoprene units. These isoprene units are not included in the biosynthesis but are used as a linkage unit from head to tail to form long hydrocarbon chains.
These terpenes can be classified on the basis of the number of the isoprene units it has. The prefix will indicate the number of isoprene units. Single isoprene units will be termed as hemiterpenes, while those having two isoprene units are termed as monoterpenes. Sesquiterpenes contains three isoprene units while diterpenes have four isoprene units and polyterpene consists of many long chains of isoprene units.
Therefore, we can conclude that all terpenes have carbon skeletons made up of isoprene units.

Hence, the correct option is A.

Note:
Terpenes are used in disciplines such as in pharmacy, biotechnology and cosmetics. It is also used as an active ingredient in pesticide to remove the insects. The formula for the isoprene unit is shown by $C{{H}_{2}}=C(C{{H}_{3}})-CH=C{{H}_{3}}$.