What's the word called part of an experiment that changes to observe an outcome?

Which of the following represents a balanced nuclear equation showing bismuth-212 undergoing alpha decay followed by beta decay? (2 points) superscript 212 over subscript 83 bi yields superscript 4 over subscript 2 he + superscript 208 over subscript 81 tl yields x−superscript 208 over subscript 82 pb + superscript 0 over subscript -1 beta superscript 212 over subscript 83 bi + superscript 0 over subscript -1 beta yields superscript 4 over subscript 2 he + superscript 208 over subscript 81 tl yields x−superscript 208 over subscript 82 pb superscript 216 over subscript 85 as yields superscript 4 over subscript 2 he + superscript 216 over subscript 85 as yields superscript 216 over subscript 85 pb + superscript 0 over subscript -1 beta superscript 212 over subscript 83 bi yields superscript 0 over subscript -1 beta + superscript 208 over subscript 81 tl yields superscript 208 over subscript 82 pb + superscript 4 over subscript 2 he