The Largest clam is

Options

A. Unio
B. Mytilus
C. Teredo
D. Tridacna.

Answer

To determine the largest clam among the given options, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are:
- A) Uneo
- B) Mytilus
- C) Pteridoc
- D) Tridecna

2. **Research Each Option**:
- **Uneo**: This is a genus of medium-sized freshwater mussels. It is not known for being large and therefore cannot be the largest clam.
- **Mytilus**: This genus includes medium to large-sized edible saltwater mussels. While they can be sizable, they are not the largest clams.
- **Pteridoc**: This genus consists of highly modified saltwater clams that bore into wood. They are not recognized as the largest clams either.
- **Tridecna**: This genus includes the species *Tridacna gigas*, which is known as the giant clam. It is the largest living bivalve mollusk.

3. **Conclusion**: Based on the research, the largest clam is *Tridacna gigas*, commonly referred to as the giant clam.

4. **Final Answer**: The correct option is D) Tridecna.