The correct explanation for why an object floats on water is:
An object that has less tightly packed atoms than water is not supported by the water underneath it.
However, this statement is poorly worded; a clearer explanation would be: "An object that is less dense (has less tightly packed atoms) than water will float because it displaces a volume of water equal to its weight."
If this option was intended to highlight that less dense objects float, you might want to clarify or select a response that correctly states that objects with lower density than water float. None of the options provided are completely accurate, but the first option about having less mass in the same volume hints toward buoyancy principles.