Final answer:
The solid structure from the options provided is Trabecula, which is found within spongy bone and serves as an internal scaffold.
Step-by-step explanation:
The question, "Which of the following is a solid structure? A) Central canal B) Lacuna C) Medullary cavity D) Trabecula", pertains to the microscopic structures within bone tissue. The correct answer is D) Trabecula. Trabeculae are the web-like, porous solid structures found within spongy bone, providing structural support much like an internal scaffold. These are distinct from the other options as the central canal, also known as the Haversian canal, contains blood vessels and nerves within an osteon; the lacuna is a small cavity that contains osteocytes; and the medullary cavity is a hollow area within the shaft of long bones filled with bone marrow.