The correct answer is: A. rationality and diminishing marginal utility.
Additivity of utility is the assumption that the utility of a combination of goods is equal to the sum of the utilities of the individual goods. This assumption is based on the idea that consumers are rational and that they will choose the combination of goods that gives them the greatest utility.
Diminishing marginal utility is the assumption that the additional utility that a consumer gets from consuming an additional unit of a good decreases as the consumer consumes more of that good. This assumption is based on the idea that consumers have limited wants and that they will eventually get to a point where they are satisfied with the amount of a good that they have.
Cardinality and independence of utility are not assumptions that are necessary for additivity of utility. Cardinality is the assumption that utility can be measured on a scale, while independence of utility is the assumption that the utility that a consumer gets from consuming a good does not depend on the consumption of other goods.
Constancy of marginal utility of money and divisibility are also not assumptions that are necessary for additivity of utility. Constancy of marginal utility of money is the assumption that the marginal utility of money is constant, while divisibility is the assumption that goods can be divided into arbitrarily small units.
In conclusion, the correct answer is: A. rationality and diminishing marginal utility.