Final answer:
A herbaceous plant with decorative foliage and scarlet inflorescence is best described as having decorative leaves and bright red flowers, which is option 3.
Step-by-step explanation:
The description of a herbaceous plant with decorative foliage and scarlet inflorescence is a plant with decorative leaves and scarlet flowers. Such a plant lacks woody stems and is characterized by its non-woody tissue.
The term 'scarlet' describes a bright red color, which directly points to the inflorescence, or the flowery part of the plant. Therefore, the correct description of such a plant would be option 3) A plant with decorative leaves and scarlet flowers.