The best choice for window material to keep the outside heat away is double-pane glass with air in between. Let`s analyze each option to understand why this is the correct answer.

Option 1: Single-pane glass is a poor choice for keeping heat out as it offers minimal insulation. It allows heat to easily transfer through the window, resulting in unwanted heat gain inside the room.

Option 2: Double-pane glass without a gap in between would provide better insulation than single-pane glass. However, without any gap or insulating material, it still allows heat transfer through conduction.

Option 3: Double-pane glass with water filled in between is not an effective choice. Water is a good conductor of heat, so it would allow heat to transfer easily from the outside to the inside, negating the purpose of insulation.

Option 4: Double-pane glass with air in between is the best choice. The air trapped between the panes acts as an insulating barrier, preventing the transfer of heat from outside to inside. Air is a poor conductor of heat, making it an effective insulator.

In conclusion, the correct choice for window material to keep the outside heat away is double-pane glass with air in between.