Explanation:

- Option 1: Thick bark

- Some plants have thick bark for protection or water conservation, but it's not unique to angiosperms.

- Option 2: Small roots

- Root size can vary greatly in plants, but isn't a distinctive feature of angiosperms.

- Option 3: Long leaf

- Leaf shape and size differ across plant species and don't define angiosperms specifically.

- Option 4: Concealed seeds

- Angiosperms are characterized by having seeds that are enclosed within a fruit.

- This unique feature distinguishes them from gymnosperms, which have exposed seeds.