The correct answer is B: during conjugation, its gametes, which are structurally and physiologically similar, fuse to form a zygote.

Here’s why:

Spirogyra is a filamentous green alga. During conjugation, the two gametes (which are similar in structure and function) do fuse, but the gametes in spirogyra do not exactly "fuse to form a zygote" directly. Instead, conjugation involves the fusion of two cells from different filaments, and this results in the formation of a zygospore after the process.

To clarify:
- A: Spirogyra is a simple multicellular plant (filamentous) and this is true.
- C: Spirogyra possesses spiral chloroplasts, which indeed helps the plant in photosynthesis.
- D: The cells of Spirogyra are surrounded by a mucilage layer, which provides protection.
- E: Pyrenoids are present in the chloroplasts of spirogyra and are involved in starch storage.

So, B is the incorrect statement because the fusion of gametes in Spirogyra during conjugation is more complex than just fusing to form a zygote; the resulting product is a zygospore, which undergoes further development.