One of the classifications of smooth muscles is based on whether the muscles act together as one unit or the muscle tissues work individually.
They are of two types: multiunit and singleunit smooth muscles.
Cells of single unit muscle act as one unit as they are interconnected. Examples are gastrointestinal tract, urogenital tract.
Cells of multiunit smooth muscles contract and relax on their own individually, and they do not act as a single unit. Examples are vascular smooth muscles, arrector pili muscle of skin, where the contraction does not get transmitted but remains confined to one particular muscle cell only which gets the direct stimulation.
So, the correct answer is D.