Final answer:
The waste material excreted by old xylem is lignin, which is found in the secondary cell walls and provides structural support to plants.
Step-by-step explanation:
The waste material excreted by old xylem is lignin. Lignin is a complex polymer found in the secondary cell walls of xylem cells and is highly resistant to degradation. It provides structural support, allowing plants to grow tall and resist the compressive forces of gravity. Over time, as the xylem cells die and form wood, this lignin remains as a waste material that is not easily broken down. Hence, the correct answer to the student's question is A) Lignin.