Choice "B" is correct. Variations in business cycle are attributed or described in terms of how long they last (duration) and the degree of peak or trough (intensity). While a recession, for example, is defined as two consecutive quarters of negative economic growth, a depression is more qualitatively assessed after consideration of both severity (depth) and duration.
Choice "c" is incorrect. The law of diminishing returns contemplates reduced output for each unit of input as industries either mature or become less efficient with size. Diminishing returns is not used to describe variations in business cycles.
Choice "d" is incorrect. Comparative advantage results in specialization by market participants and does not describe variations in the business cycle.
Choice "a" is incorrect. Opportunity costs represent implicit costs that evaluate the cost of alternate uses of resources. Business cycles are not attributed to opportunity costs.