The correct answer is C. equal.
A consumer is in equilibrium when the marginal utilities from both goods are equal. This means that the consumer is getting the same amount of satisfaction from each good. If the marginal utility from one good is higher than the marginal utility from the other good, the consumer will want to consume more of the good with the higher marginal utility. Conversely, if the marginal utility from one good is lower than the marginal utility from the other good, the consumer will want to consume less of the good with the lower marginal utility.
Option A is incorrect because the marginal utility cannot be greater than the maximum possible positive value. The maximum possible positive value is the amount of satisfaction that a consumer can get from consuming one more unit of a good.
Option B is incorrect because the marginal utility cannot be less than the minimum possible positive value. The minimum possible positive value is the amount of satisfaction that a consumer can get from consuming one more unit of a good even if they are not consuming any of the good.
Option D is incorrect because the marginal utility cannot be zero. The marginal utility is the amount of satisfaction that a consumer gets from consuming one more unit of a good. If the marginal utility is zero, then the consumer is not getting any satisfaction from consuming more of the good.