Final answer:
When a company lengthens a product line by adding more items within the current range, it is called filling the line, which adds related products to cover gaps in the range.
Step-by-step explanation:
When a company lengthens a product line by adding more items within that line's current range, they are filling the line.
This strategy involves adding new products that are related to the existing products but fill gaps in the current range, either by meeting additional customer needs, targeting different price points, or offering different features.
It does not involve adjusting the existing products' quality or price levels, which would be considered stretching the line either upward or downward, nor does it affect the width of the product mix (which refers to the number of product lines a company offers) or the depth of the line (which refers to the number of variations offered within a single product line).