The most primitive bilaterally symmetrical organism is :-

Options

A. Ascaris
B. Fasciola
C. Leucosolenia
D. Hydra

Answer

To determine the most primitive bilaterally symmetrical organism from the given options, we will analyze each option based on their classification and characteristics.

### Step-by-Step Solution:

1. **Understanding Bilateral Symmetry**:
- Bilateral symmetry refers to organisms that can be divided into two identical halves along a single plane. This is a characteristic of more advanced organisms compared to those with radial or asymmetrical symmetry.

2. **Analyzing the Options**:
- **Option 1: Ascaris**
- Ascaris is a nematode (roundworm) and exhibits bilateral symmetry. However, nematodes are more advanced than some other bilaterally symmetrical organisms.

- **Option 2: Fasciola**
- Fasciola, also known as the liver fluke, is a flatworm (class Trematoda). Flatworms are among the simplest bilaterally symmetrical organisms and are considered more primitive than nematodes. They have a simple body structure with only one opening to the gut and lack a coelom.

- **Option 3: Leucosolenia**
- Leucosolenia is a sponge, which belongs to the phylum Porifera. Sponges are asymmetrical and do not exhibit bilateral symmetry, making them irrelevant to this question.

- **Option 4: Hydra**
- Hydra belongs to the phylum Cnidaria and exhibits radial symmetry rather than bilateral symmetry. Therefore, it cannot be considered in this context.

3. **Conclusion**:
- Among the options provided, Fasciola (the liver fluke) is the most primitive bilaterally symmetrical organism. It is simpler in structure compared to Ascaris and is classified as a flatworm.

### Final Answer:
The most primitive bilaterally symmetrical organism is **Fasciola**.