When the probabilities are not known, decision making can be based on various methods, including optimistic, pessimistic, opportunistic, and decision criteria. Optimistic decision making involves assuming the best possible outcome, while pessimistic decision making involves assuming the worst possible outcome. Opportunistic decision making involves taking advantage of any available opportunity, regardless of the potential risks or rewards. Decision criteria can be used to evaluate different options based on specific criteria, such as cost$,$ time, or feasibility. EMV $($Expected Monetary Value$)$ is a decision$-$making tool that helps compare the overall impact of different situations. It is a relative measure of preferability among alternatives, as it compares the expected value of each alternative against a reference alternative. The expected monetary value is calculated by multiplying the probability of each outcome by the associated monetary value and then summing up the results. This allows decision makers to evaluate different decision alternatives based on their potential outcomes and associated risks and rewards. Final answer: In summary, when the probabilities are not known, decision making can be based on various methods, including optimistic, pessimistic, opportunistic, and decision criteria. These methods help individuals and organizations make informed decisions in the absence of complete information. EMV is a decision$-$making tool that helps compare the overall impact of different situations by providing a relative measure of preferability among alternatives, which can be useful in evaluating different decision alternatives.