The correct answer is option D) Lumen of Intestine.

Explanation:
Septal nephridia are excretory organs found in earthworms. They are located in each segment of the earthworm's body, specifically in the septa (partition walls) that separate each segment. These nephridia play a vital role in the excretion of metabolic waste from the earthworm's body.

Septal nephridia function in a similar way to other nephridia found in earthworms. They consist of a ciliated funnel-like structure called a nephrostome, a tubule, and a bladder-like structure called a nephridial bladder. The nephrostome collects waste materials and excess fluids from the coelomic fluid (fluid found in the body cavity of the earthworm).

The collected waste materials and excess fluids then pass through the tubule of the septal nephridium. Along the tubule, reabsorption of useful substances like water, glucose, and amino acids occurs. After reabsorption, the remaining waste products and fluids are transported to the nephridial bladder.

The nephridial bladder acts as a storage organ for the excretory products. From the nephridial bladder, the waste materials are discharged into the lumen of the intestine. The intestine is a long, coiled tube that runs through the length of the earthworm's body. It is responsible for the digestion and absorption of nutrients.

By discharging the excretory products into the lumen of the intestine, the earthworm can eliminate waste materials along with undigested food particles. This process ensures efficient excretion and helps maintain homeostasis by removing metabolic waste from the earthworm's body.

In conclusion, septal nephridia of earthworms discharge excretory products into the lumen of the intestine, allowing for the elimination of waste materials along with undigested food particles.