The correct answer is option 4 - Double-pane glass with air in between.

Option 1 - Single-pane glass: Single-pane glass is not an efficient choice for keeping outside heat away as it does not provide good insulation. It is the least effective option in terms of energy efficiency.

Option 2 - Double-pane glass without a gap in between: While double-pane glass is an improvement over single-pane glass, without a gap in between the panes, it offers limited insulation benefits. Heat can still easily transfer through the glass, making it less effective at keeping outside heat away.

Option 3 - Double-pane glass with water filled in between: Water-filled double-pane glass is not a common or practical choice for window material. Water can freeze, expand, and possibly damage the glass. Additionally, water is not an effective insulator compared to other materials.

Option 4 - Double-pane glass with air in between: Double-pane glass with air in between is the best choice for keeping outside heat away. The air trapped between the panes acts as an insulator, reducing heat transfer through the window. This helps to keep the interior of the building cooler in hot weather, leading to energy savings and improved comfort.