Final answer:
The correct variables representing the pressure, volume, and temperature of a van der Waals gas with n molecules are n, P, V, and T. Hence, the correct option is 4) n, p, t.
Step-by-step explanation:
The variables that represent the pressure, volume, and temperature of a van der Waals gas with n molecules are n, P (pressure), V (volume), and T (temperature). The ideal gas law for a van der Waals gas can be represented by the formula PV = nRT, where P stands for pressure, V is volume, n is the number of moles, T is temperature in Kelvin, and R is the ideal gas constant. Therefore, the correct answer to the question 'What are the variables that represent the pressure, volume, and temperature of a van der Waals gas with n molecules?' is option 4) n, p, t.