Assertion (A) It is necessary to study life history of an ascidian to consider it as chordate
Reason (R) Tadpole larva of ascidians have a notochord that disappears in the adult

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not the correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that it is necessary to study the life history of an ascidian to consider it as a chordate.
- Ascidians, also known as sea squirts, are marine animals that belong to the subphylum Urochordata within the phylum Chordata.

2. **Understanding the Reason (R)**:
- The reason provided states that the tadpole larva of ascidians has a notochord that disappears in the adult form.
- The presence of a notochord is a key characteristic of chordates. In ascidians, the larval stage (tadpole) exhibits this feature, but it is lost in the adult stage.

3. **Connecting Assertion and Reason**:
- To classify an organism as a chordate, it is essential to observe its developmental stages, particularly the presence of the notochord during the larval stage.
- Since the larval form of ascidians has a notochord, this supports their classification as chordates, but the adult form does not retain this structure.

4. **Evaluating the Truth of A and R**:
- Both the assertion and the reason are true. The assertion is true because understanding the life history is crucial for classification. The reason is also true because the presence of a notochord in the larval stage is a defining characteristic of chordates.

5. **Determining the Relationship Between A and R**:
- The reason (R) provides a correct explanation for the assertion (A). The need to study the life history is directly linked to the presence of the notochord in the larval stage of ascidians.

6. **Conclusion**:
- Therefore, the correct answer is that both A and R are true, and R is the correct explanation of A.

### Final Answer:
- **Option 1**: A and R are true, and R is the correct explanation of A.

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