Thermal paste is the correct answer because it is specifically designed to fill the microscopic gaps and imperfections between the CPU and the heat sink, improving thermal conductivity and heat transfer efficiency. A heat pad might provide some level of heat transfer but generally isn't as effective as paste due to its solid nature and possible thickness. Both isopropyl alcohol and lubricating gel do not serve as thermal interface materials; isopropyl alcohol is used to clean the CPU surface, and lubricating gel would actually inhibit effective heat transfer and could potentially cause damage.