The correct answer is: B. Probability
Risk in capital budgeting implies that the decision-maker knows the probability of the cash flows. This is because the decision-maker needs to be able to assess the likelihood of different outcomes in order to make an informed decision about whether or not to invest in a project.
Option A, variability, is not the correct answer because it does not take into account the probability of different outcomes. Option C, certainty, is also not the correct answer because it assumes that the decision-maker knows with certainty what the cash flows will be. This is not always the case, as there is always some degree of uncertainty involved in any investment decision.
Option D, none of the above, is also not the correct answer because it does not include the correct answer, which is B. Probability.