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The argument draws a causal conclusion (make the module mandatory → boost completion) from an observed correlation (those who choose it finish more often). A key threat is selection bias: students who voluntarily complete an optional module may be more motivated or better prepared. If higher completion is due to those preexisting traits, mandating the module will not replicate the effect. Option A precisely articulates the required assumption that the module itself drives the improvement, not self-selection. Option B concerns implementation efficiency, not the logic of the claim. Option C introduces an equity issue that could matter operationally but is not logically necessary for the causal inference. Option D is peripheral; instructor training may be useful but is not the linchpin assumption. On GRE Argument tasks, always scrutinize whether an intervention’s observed benefit among volunteers truly generalizes; ruling out selection effects is often the necessary bridge from correlation to causation.