In statistics, the basic property of density curves states that they must lie:

In statistics, density curves represent the probabilities associated with continuous random variables. A fundamental property of density curves is that they must lie above the x-axis. This is because the height of the curve at any point represents a probability density; thus, it cannot be negative, as probabilities themselves are always non-negative.

If a density curve were to dip below the x-axis at any point, it would imply a negative probability density, which is not meaningful in probability theory. The area under the curve must also sum to one, as this represents the total probability for all possible outcomes of the random variable. Therefore, maintaining a position above the x-axis is essential for the correct representation of probabilities in a density curve.

The choice that suggests the curve must lie above the y-axis is not relevant since the density is a function of the values of the random variable (x-axis), and negative heights for probabilities are not permitted in this context. Thus, it is correct to affirm that density curves must lie above the x-axis.