Correct answer is (a) Metal species released
To explain I would say: Metal species released after death of the bacteria that has metal accumulation is compared less toxic than the actual free metal species released in the environment. If the metal is not assimilated by the bacteria and released in the environment, it shows higher toxic potential. After the death of the bacteria, the microorganisms produce aggregates that form complexes with the metals and hence, released into the surrounding after the death of the bacteria.