What best describes a chain reaction in chemical processes?

A chain reaction in chemical processes is best described by the characteristic where every reactive species results in the formation of a new species. This definition highlights the self-perpetuating nature of chain reactions, where the products of one step can initiate further reactions, creating a sequence that continues as long as reactants are present.

In many chain reactions, such as those seen in polymerization or nuclear fission, each reaction step produces new reactive intermediates which can react further, leading to an exponential increase in products. This underscores the principle that the process is driven by the formation of additional reactive species which continue the chain.

The other options do not capture this fundamental feature. While a chain reaction may indeed generate heat or occur at high temperatures, these attributes are not defining characteristics of the reaction itself. Additionally, the consumption of all reactants does not apply to many chain reactions, as they often leave some reactants unreacted or regenerate some of the initial species during the process. Thus, the emphasis lies in the generation of new species driven by the reactions, which is why the choice highlighting the formation of new species is the most accurate description of a chain reaction.