Fertilizer

To solve the problem, we need to identify which of the organisms listed undergo external fertilization.

1. External Fertilization:
External fertilization occurs when the fertilization of eggs by sperm takes place outside the female's body. This is commonly observed in aquatic animals like fish and amphibians. In external fertilization, the eggs and sperm are released into the surrounding environment (typically water) for fertilization.

2. Analyzing the Options:
- Monkeys and humans undergo internal fertilization, where fertilization occurs inside the female's body.
- Fish are known for external fertilization, where fertilization happens in the water after both eggs and sperm are released externally.

Final Answer:
External fertilization is observed in .