B: anything that decreases the mundane realism of the study

    Incorrect.

Mundane realism is defined as the degree to which the experiment resembles real life.  Selection biases are certain actions taken that threaten the selection procedures of an experiment (random assignment).  Both mundane realism is a threat to external validity not a  threats to internal validity.  This answer is incorrect, simply because mundane realism is NOT a selection bias, or vice versa.