The answer depends on the specific context of the economy and how the production of gasoline is being allocated.
a. The marginal benefit of the 100th gallon is bigger than the marginal benefit of the 5th gallon is not always true. The marginal benefit of a good is the additional benefit that a consumer receives from consuming one more unit of the good. As such, the marginal benefit of gasoline can vary depending on a consumer's needs and preferences. For example, if a consumer is stranded on a highway with no gas, the marginal benefit of the 5th gallon may be much higher than the marginal benefit of the 100th gallon.
b. The marginal benefit of the 100th gallon is smaller than the marginal benefit of the 5th gallon is not always true. This is similar to the point I made above. The marginal benefit of a good can depend on a consumer's needs and preferences, so it is not possible to say that the marginal benefit of the 100th gallon will always be smaller than the marginal benefit of the 5th gallon without more information about the specific context.
c. The opportunity cost of producing the 100th gallon is smaller than the opportunity cost of the 5th gallon is also not always true. Opportunity cost is the cost of foregone opportunities, i.e., the next best alternative that must be given up in order to pursue a certain action. The opportunity cost of producing gasoline can depend on various factors such as the availability of resources, the cost of production, and the demand for other goods. So, its not possible to say that the opportunity cost of producing the 100th gallon is always smaller than the opportunity cost of the 5th gallon without more information about the specific context.