Final answer:
The most reasonable conclusion from the observation that only the surface of the water in a beaker boils is that water is a poor conductor of heat, which leads to inefficient heat transfer through its body. So the correct option is a.
Step-by-step explanation:
If a beaker of water is placed under a broiler, with the observation that only the surface layer boils while the water at the bottom of the beaker remains close to the initial temperature, it can be concluded that water is a poor conductor of heat. This phenomenon happens because heat is not efficiently transferred through the body of the water from the hot surface at the top to the cooler sections below. In this scenario, the heat from the broiler predominantly warms the water by radiation directly at the surface, but the lack of conduction prevents the lower part of the water from reaching the boiling point.
Heat conduction in materials depends on the kinetic energy transfer between molecules. Good conductors, like metals, allow for a rapid heat flux due to the effective transfer of energy during molecular collisions. In contrast, poor conductors experience less efficient energy transfer, resulting in a slower rate of heat flow. Additionally, convection could play a role in heat distribution, but in this case, the observation that only the surface boils suggests conduction is the limiting factor for heat transfer to the bottom of the beaker.