In a fission reaction, a heavy nucleus splits into smaller nuclei, releasing energy and often neutrons. Therefore, to identify a fission reaction, you would typically see a nucleus with a large mass on the left side (the reactants) and smaller nuclei, often with neutrons produced, on the right side (the products).
From the provided responses, the correct identification would be:
A nucleus with a large mass will be on the left side, and there will be neutrons in the equation.