Can you tell me what to include in a model of ecosystem for a school project? for the 6th grade students what is good? Thanks for your helps!

I’m trying to help my daughter and these are the multiple choice answers:
A. Another species will become the keystone species.
B. A keystone species from another ecosystem may take over.
C. Most species in the ecosystem will be severely affected.
D. The ecosystem will not change.

what will happen to the other organisms in the food web if all frogs in the ecosystem were killed?