Some organisms are not able to live in an environment where there is oxygen; these types of organisms are called obligate anaerobes. which explanation is most plausible for how they survive without oxygen?

Microorganisms which are killed in the presence of normal concentrations of atmospheric oxygen are called obligate anaerobes.Some species are capable of surviving in upto 8% while other losing viability in 0.5%.They survive without oxygen as they use other molecules as electron acceptor instead of oxygen.

Explanation:In aerobic organisms oxygen is used in cellular respiration as an electron acceptor.oxygen accepts electrons and turns into water H2O or CO2.While obligate anaerobes other molecules as their primary electron acceptors.common electron acceptors used by them are Nitrate NO3-,  Sulphate SO42- or sulphur S,Iron,mercury and manganese.So in this way they undergo respiration and survive without oxygen.

a corroding iron, if that's one of your choices

plasma - state of matter made up of electrically charged particles