A group of students was studying development of an organism under controlled laboratory conditions. Following observations were made by them.
i. The laravel had a rod-like supporting structure that separated the nervous system and the gut.
ii. A prominent central cavity was present in the transverse section of the part of the nervous system of the larvae, while the adults had cerebral ganglia as the main component of the nervous system.
iii. The eye were prominently seen in larvae.
iv. The tails were absent in the adults, which the larvae had.
v. A lot of phagocytic activity was observed before conversion of larvae into adults.
vi. The adults had a cuticular exoskeleton.
The organism understudy must be belonging to,

Options

A. Amphibia
B. Pisces
C. Protochordata
D. Arthropoda

Explanation

To determine the classification of the organism based on the observations provided, we can analyze each observation step by step.

### Step-by-Step Solution:

1. **Observation of Larval Structure**:
- The larva has a rod-like supporting structure that separates the nervous system and the gut. This structure is indicative of a notochord, which is a characteristic feature of chordates.

**Hint**: Look for structures that indicate the presence of a notochord, as this is a key characteristic of chordates.

2. **Nervous System Development**:
- A prominent central cavity is present in the transverse section of the nervous system of the larvae, while adults have cerebral ganglia. This suggests a transition from a simpler nervous system in larvae to a more complex one in adults.

**Hint**: Consider how the nervous system evolves from larval to adult stages, which can indicate the organism's classification.

3. **Presence of Eyes**:
- The presence of prominent eyes in the larvae suggests a level of sensory development typical in many chordates.

**Hint**: Sensory structures can provide clues about the organism's classification, especially in relation to its developmental stages.

4. **Tail Presence**:
- The larvae have tails, which are absent in adults. This is a common feature in many chordate life cycles, where the larval stage often has a tail that is lost in adulthood.

**Hint**: Note the changes in body structures between larval and adult forms, as this can help narrow down classifications.

5. **Phagocytic Activity**:
- A lot of phagocytic activity observed before the conversion of larvae into adults indicates a significant metabolic and developmental change, typical in organisms undergoing metamorphosis.

**Hint**: Metamorphosis is a key process in many chordates, especially in amphibians and some protochordates.

6. **Cuticular Exoskeleton**:
- The adults have a cuticular exoskeleton. This feature is more characteristic of arthropods, but given the context of the other observations, it may suggest a different classification.

**Hint**: While exoskeletons are common in arthropods, consider how the other characteristics align with the organism's classification.

### Conclusion:
Based on the observations, the organism under study must belong to **Protochordata**. This group includes animals that are closely related to vertebrates and possess characteristics such as a notochord and a simple nervous system in their larval stages.

### Final Answer:
The organism under study must be belonging to **Protochordata**.