Explanation:

- Option 1: Thick bark

- Many trees, both angiosperms and gymnosperms, have thick bark for protection, making it not distinctive to just angiosperms.

- Option 2: Concealed seeds

- Angiosperms uniquely produce seeds that are enclosed within an ovary, often becoming a fruit. This is their hallmark feature.

- Other plant groups, like gymnosperms, have exposed seeds (e.g., pine cones).

- Option 3: Long leaf

- Leaf length varies widely among angiosperms. While some have long leaves, it is not a universal trait.

- Option 4: Small roots

- Root size can vary greatly among angiosperms, with many having large roots. This is dependent on species and environment rather than being a distinctive feature.

Correct Answer: Option 2: Concealed seeds