Explanation:

- Option 1: Anisogamous

- In anisogamy, gametes of different sizes or forms fuse. Typically, one gamete is larger, which is often termed the egg, while the smaller one is motile and called the sperm.

- Seen in many algae and some other organisms.

- This is the correct option for Eudorina.

- Option 2: Oogamous

- Similar to anisogamy but usually implies a more pronounced size difference.

- Typically, one large non-motile egg and a smaller motile sperm.

- Option 3: Zoogamous

- This term is not commonly used in scientific literature to describe gamete fusion.

- It generally refers to organisms that fertilize via animal vectors.

- Option 4: Isogamous

- Gametes are similar in size and shape.

- Commonly seen in some algae and fungi.