The correct answer is: B. Within the middle third of the base.
The supporting area of an offset footing of a boundary wall is fully compressive if the C.G. of load acts within the middle third of the base. This is because the middle third of the base is in compression, while the outer thirds are in tension. If the C.G. of load acts outside the middle third, the outer thirds will be in tension, which can cause the footing to fail.
Option A is incorrect because the C.G. of load must act within the middle third of the base, not at the center of the base. Option C is incorrect because the C.G. of load must act within the middle third of the base, not within the middle fifth of the base. Option D is incorrect because the C.G. of load must act within the middle third of the base.