The more the image resolution would be the more storage it needs. If you think about it when a website tries to load a large image it takes longer than loading a smaller one. Hence, if you have a slow telephone line and need your images to be displayed (loaded) faster, then you have to go with the smallest resolution (the smallest storage).
In a nutshell, your answer will be d) 320x240
Note: 320 x 240 is the dimensions of the image (width x height) and those numbers are measured in pixels which is the unit of measuring images, display screens,...etc. i.e. this image is 320 pixels in width and 240 pixels in height