Opportunity cost and explicit cost are two different ideas that this question is testing. Explicit costs are actual cash payments made because of a decision, while opportunity cost is what you give up by not choosing an alternative. Education, self-employment and leisure are all things a person could have done instead of taking a paid job, so foregoing them counts as opportunity cost. Commuting expense, however, is money spent as a direct result of going to work, similar to any other job-related expense, so it belongs to explicit cost rather than opportunity cost. \[\boxed{\text{Transportation costs of commuting to work}}\]