Here's a problem to solve: A plant grows in a damp, marshy area filled with nonvascular plants and seedless vascular plants. Which characteristic indicates the plant is classified as a seedless vascular plant?

Among the 4 following options, which is the correct answer?

- A: It is 18 cm tall.
 
- B: It produces spores.
 
- C: It grows in poor soil.
 
- D: It has a flat gametophyte.
 