The correct answer is C. Constant marginal utility of money.
The theory of demand is based on the assumption that consumers have a diminishing marginal rate of substitution. This means that as a consumer consumes more of one good, they are willing to give up less of another good in order to get an additional unit of the first good.
The assumption of constant marginal utility of money would imply that consumers are always willing to give up the same amount of money in order to get an additional unit of a good. This is not a realistic assumption, as consumers are generally more willing to give up money for goods that they value more highly.
The other options are all assumptions of the theory of demand. Option A states that consumers have a given scale of preferences as between different combinations of two goods. This means that consumers can rank different combinations of goods in order of preference. Option B states that the marginal rate of substitution is diminishing. This means that as a consumer consumes more of one good, they are willing to give up less of another good in order to get an additional unit of the first good. Option D states that consumers would always prefer more of a particular good to less of it, other things remaining the same. This means that consumers are always willing to consume more of a good if their income increases or the price of the good decreases.