If you are "mortal", it means that you can die. We are all mortal -- humans live on average a little over 70 years, although some live to be 120.
If you are "immortal", it means you cannot die. Gods are immortal, for example.
If something is "lethal", it means it can kill you. "Lethal injection" is a possible method of execution: the prisoner is given an injection which contains poisons.
If something is "benign", it means it's not very dangerous. If you have a benign tumor, you can be happy, because it usually means your life is not in danger.
Answer A would mean that environmental pressures can make microbes either mortal (they will die) or immortal (they will never die). That's not true: environmental pressures can make microbes either lethal (they will kill you) or benign (they will not kill you).