Explanation:

- Generous: Typically refers to someone who is willing to give more of something, like time or money, than is usual or expected. It isn't used to describe a physical object like a souvenir.

- Bountiful: Often used to describe abundance, usually in a harvest or supply context. It's not commonly used to describe souvenirs.

- Considerable: Refers to something significant in amount or degree. While it could work for the size or impact of a souvenir, it's not the best fit.

- Voluminous: This word means taking up much space; large in volume. It's appropriate for describing a substantial or large souvenir.

Therefore, option 4 - "voluminous" is the correct answer.