Explanation:

Correct option 3: parasite : host

- The analogy is about the relationship between the first and the second:

- A bacterium lives and grows in a medium (an environment or substance).

- Let's review the options:

1. meadow : grass — The grass grows in a meadow, but the relationship is not parallel.

2. snake : egg — Snakes hatch from eggs, but they don’t "live in" eggs as a habitat.

3. parasite : host — A parasite lives in/on its host, just as a bacterium lives in a medium.

4. predator : den — Predators may live in dens, but the relationship is not as direct as parasite-host.

- The best parallel is Option 3: parasite : host, because both involve living inside/on a sustaining environment or organism.