as the solar system was forming about 4.5 billion years ago, earth was being bombarded by small-diameter rocks,
comets, and debris. what effect did this debris likely have on earth's early surface?

The debris had some effects on the earth.

Explanation:

During the younger years of earth it was subjected to the effects of various asteroid hits and attack of leftovers from failed planets. The period of the universe before 4.5 billion years ago to 3.8 billion years ago was one of vigorous activity. The terminal period is known as Late Heav Bombardment and it was the period of most violent activities.

The attacks from asteroids and failed planets caused cracks and scars on the surfaces of several planets. The impact is more visible in the case of small planets. Signs of the attacks in the LHB can also be found on earth but they have become less visible over the years due to weathering process that takes place on earth.

Signs of collisions can be clearly seen in planets like Mercury, Venus and Mars which are rocky planets. Moon also has several signs of collision. It is also suggested that the water on earth came from asteroids that hit the earth. The atmosphere of the earth prevented this water from escaping.  

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.