We have the following information:
1. x is the garden's original length.
2. The area of the garden is the expression 9 * (x + 2) (the length is extended 2 feet).
3. 9 is the width of the garden.
When we expand the expression, we have that:
9 * (x + 2) = 9x + 9*2 = 9x ft^2 + 18 ft^2.
Then, we have that the area of the extension is 18 square feet (we multiply the width by the extension, 2 feet).
Therefore, the correct option is C (9x + 18); 18 square feet.