**Introduction:**
Earthworms are segmented worms belonging to the phylum Annelida. They have a well-developed body plan with distinct segments. Each segment of an earthworm is covered with small bristle-like structures called setae, which help the worm in locomotion and burrowing. However, there are certain regions of the earthworm's body where setae do not occur.

**Explanation:**
The correct answer is option 'D' - all of the above. Let's examine each region mentioned in the options:

**a) Prostomium (presegmental region) and peristomium (first segment):**
The prostomium is the anterior part of the earthworm's body, which is not segmented. It acts as a sensory organ and helps in the detection of food. The peristomium is the first segment, which surrounds the mouth. In both these regions, setae do not occur. This is because these regions are not involved in locomotion or burrowing. The setae are primarily located on the ventral side of the earthworm's body segments, where they aid in movement through the soil.

**b) Clitellum (14-16 segments):**
The clitellum is a swollen, glandular region present in certain segments of the earthworm's body. It is involved in the reproduction of earthworms. Setae are absent in this region as well. This is because the clitellum is not involved in locomotion or burrowing but rather in the production of a cocoon for the deposition of eggs.

**c) Anal segment:**
The anal segment is the last segment of the earthworm's body, located at the posterior end. It contains the anus through which waste material is eliminated. Setae are not present in this region either. The absence of setae in the anal segment allows for the easy passage of waste material without obstruction.

**Conclusion:**
In conclusion, setae do not occur in the regions mentioned in all the options (a, b, and c). The absence of setae in these regions is related to their specific functions and not requiring the aid of setae for locomotion or burrowing.

C