Identify the difference between single-unit and multi-unit smooth muscles from the given options.

Concept: Smooth muscles are a type of involuntary muscles that are found in the body of the organism. They are not under the control of any force in the body and can function on their own. Smooth muscles are further divided into two categories depending on their presence either in solitary way or in the form of clusters. These categories are single unit muscles and multi unit muscles. 

Single unit muscles are connected to each other with the help of gap junctions whereas in multi unit muscles these gap junctions are not present. Single unit muscles act together as a single functional unit whereas multifunctional muscles act as separate units. Single unit muscles have a syncytium present and they can get affected by the stretching of the muscles. Multi unit smooth muscles do not have syncytium present and they are not affected by the stretching of the muscles. Single unit smooth muscles are found in the stomach and the bladder and the multiunit smooth muscles are found in eyes and in blood vessels. 

Hence, the correct option is D.