Which alga reproduce sexually by conjugation ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Sexual Reproduction in Algae**: Sexual reproduction in algae can occur through various methods, one of which is conjugation. This process involves the fusion of two algal cells to exchange genetic material.

2. **Identifying the Alga**: Among the various types of algae, we need to identify which one specifically reproduces through conjugation.

3. **Recognizing the Process of Conjugation**: In conjugation, two filaments of algae align and form a conjugation tube. This allows the transfer of genetic material between the two cells.

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