Final answer:
Leydig cells have a well-developed smooth endoplasmic reticulum and are responsible for the production of the steroid hormone testosterone.
Step-by-step explanation:
The steroid-producing cells that have a well-developed smooth endoplasmic reticulum (SER) are Leydig cells (option D). These cells are located between the seminiferous tubules in the testes and are responsible for testosterone production. Leydig cells contain a high concentration of SER, which is essential for the synthesis of steroid hormones like testosterone. While Sertoli cells (option C) do play an important role in sperm production and have functions related to supporting germ cells, they are not primarily known for steroid hormone synthesis. Pancreatic cells (option A) mainly produce digestive enzymes and hormones like insulin and glucagon, and adipose cells (option B) store fat and secrete adipokines, but neither are known for a significant role in steroid hormone synthesis compared to Leydig cells.