Here are the answers to your questions:
Question 1:
A) Which phrase best describes nuclear fusion?
Answer: the process by which small nuclei combine into a larger nucleus
Question 2:
A) Which phrase describes a nuclear chain reaction?
Answer: a series of reactions in which particles from one reaction trigger the next reaction
Question 3:
A) Which option would be an appropriate model of nuclear fission?
Answer: cutting a cake in half
Question 4:
A) The process of radioactive decay is unpredictable and irreversible. Which option would be an appropriate model of the radioactive decay of a group of atoms?
Answer: petals falling from a flower
Question 5:
A) Which statement best describes the effect of radioactive decay on a nucleus?
Answer: The resulting nucleus is less stable than the original nucleus. The nucleus can be of a different element than the original.