The type of sexual reproduction in Spirogyra is

The correct option is isogamous.

Detailed Explanation:
Sexual reproduction in Spirogyra (a genus of filamentous green algae) occurs through a process known as conjugation. During this process, the mode of sexual reproduction is classified as isogamous based on the morphological characteristics of the fusing gametes.

Here is a step-by-step breakdown of how sexual reproduction occurs in Spirogyra:
1. Morphological Similarity (Isogamy): The fusing gametes are morphologically identical. They are similar in size, shape, and structure, and lack flagella. Because both gametes look identical, they are called isogametes.
2. Non-motile Gametes: The gametes of Spirogyra do not have flagella and are therefore non-motile (aplanogametes). The transfer of the active gamete to the passive gamete occurs physically through a conjugation tube formed between two filaments.
3. Physiological Anisogamy: Although morphologically identical (isogamous), they exhibit a division of labor. One gamete acts as the active partner (male gamete) by migrating through the conjugation tube, while the other remains passive (female gamete) inside its cell.

Understanding the other options:
• Anisogamous: This involves the fusion of gametes that are morphologically different, typically differing in size.
• Oogamous: This involves the fusion of a small, active, motile male gamete with a large, passive, non-motile female gamete (egg).
• Cleistogamous: This term refers to a mechanism of self-pollination occurring within closed flowers in flowering plants (angiosperms), which is not applicable to algae.