Closed car with the windows rolled up heats up on sunny days because of

global warming

the greenhouse effect

convection currents

conduction

the greenhouse effect

Explanation:

The greenhouse effect can be define as the trapping of the heat energy coming from the sunlight by the gases present in atmosphere. This makes the atmospheric temperature warm and fluctuates the seasonal and climatic cycles. This occurs due to the abundance of greenhouse gases such as water vapor, carbon dioxide, carbon monoxide, sulphur dioxide, nitrous oxides and methane. Generally the heat energy from the terrestrial regions get reflected back into the space this maintains the heat balance on earth. But due to the abundance of greenhouse gases into the atmosphere this heat energy does not reflect back to the space. Resulting in greenhouse affect as the heat get trapped in the atmosphere which will affect negatively to the other spheres of the earth.  

The closed car is the example of greenhouse effect. This is because of the fact that the closed car window pane will absorb the heat energy incident-ed on it. This will make the internal temperature of the car warm just like the warm temperature caused by the greenhouse effect of earth.