A: a large event, such as an earthquake, that happens in the middle of the study

    Incorrect.
An earthquake in the middle of an experiment COULD threaten internal validity; however, this situation would not create a selection bias.  We can imagine how the presence of an earthquake may allow us to question whether the independent variables (causes) had an effect on the dependent variables (effects), because a confounding variable is present, in this case, the earthquake.  Make sure you read the question carefully!  In this case the question asks specifically about selection biases.  Choice A is incorrect, because a selection bias may occur when random assignment procedures have been disrupted.  Typically, this would involve certain subjects being chosen over others, or a situation in which subjects select their own experimental condition.  An earthquake in the middle of the study would not do this.