A student provides four conditions that a function must satisfy in order to be a probability density function. However, only one of the conditions is actually correct. Of the following four statements, the one that accurately describes a property that a function must possess in order to be a probability density function is:

a) The set of values along the horizontal axis that the function assigns values to must be the set of values between 0 and 1.

b) The function must not take values greater than 1.

c) The function must not go below the horizontal axis at any point.

d) The area between the function and the horizontal axis must be less than or equal to 1.