The wall is 9 feet by 12 feet, so its area is given by,
[tex]\begin{gathered} =9\cdot12 \\ =108 \end{gathered}[/tex]So the area of the wall that needs to be painted is 108 square feet.
Given that each 's' square foot required 1 gallon of paint, then the amount of paint required to paint the complete wall will be calculated as,
[tex]\begin{gathered} \because s\text{ sq ft}\equiv1\text{ gallon} \\ \therefore108\text{ sq ft}\equiv\frac{108\cdot1}{s}=\frac{108}{s}\text{ gallons} \end{gathered}[/tex]So the amount of paint required for the whole wall is 108/s gallons.
Given that 1 gallon of paint costs 'g' dollars, the cost of total paint required will be,
[tex]\begin{gathered} \because1\text{ gallon paint}\equiv g\text{ dollars} \\ \therefore\frac{108}{s}\text{ gallons paint}\equiv\frac{(\frac{108}{s})\cdot g}{1}=\frac{108g}{s}\text{ dollars} \end{gathered}[/tex]Thus, the cost (in dollars) of the total paint required for painting the wall is obtained as,
[tex]\frac{108g}{s}[/tex]Therefore, option D is the correct choice.