What distinguishes a saturated hydrocarbon from an unsaturated hydrocarbon?

The distinction between saturated and unsaturated hydrocarbons is primarily based on the types of bonds present in their molecular structure. Saturated hydrocarbons are characterized by having only single bonds between carbon atoms, which means they are fully "saturated" with hydrogens. This structure allows them to hold the maximum number of hydrogen atoms possible, as no double or triple bonds are present to reduce the hydrogen count.

On the other hand, unsaturated hydrocarbons contain one or more double or triple bonds between carbon atoms. These bonds create points where fewer hydrogen atoms can be attached due to the nature of the bonding. Therefore, the defining feature of saturated hydrocarbons is their lack of double or triple bonds, which allows for the highest saturation of hydrogen.

In this context, the other choices do not accurately describe saturated hydrocarbons. Saturated hydrocarbons are not characterized by double bonds or a smaller number of hydrogen atoms, nor are unsaturated hydrocarbons exclusively cyclic. Thus, the correct understanding is that saturated hydrocarbons are defined by the absence of double or triple bonds, making them fully saturated with hydrogen.