Pioneer plant species could be described as those that,

A) sprout from seeds after a forest fire.

B) form large forest trees that shade understory plants.

C) are slow to mature, and require a soil with well-balanced nutrients.

or

D) are small in size, and have highly effective means of dispersing their seeds.

The correct answer is option A, sprout from seeds after a forest fire

Explanation:

Pioneer species are basically the first species that grow or re-originate in any area after a disturbance. Disturbances may include forest fires, floods, etc.

Here in this questions, in option A, seeds are sprouting after forest fire. Forest fire is a disturbance and seeds sprouting just after a disturbance can be considered as pioneer species of plant.

D) are small in size, and have highly effective means of dispersing their seeds.