The correct answer is $\boxed{\frac{1}{2}}$ horizontal to 1 vertical.
The spread from the wall base to outer edge of a brick work foundation is the distance between the wall base and the outer edge of the foundation. This distance is important because it determines the amount of weight that the foundation can support. The spread should be no more than $\frac{1}{2}$ horizontal to 1 vertical, or 0.5:1. This means that the distance from the wall base to the outer edge of the foundation should be no more than half the width of the foundation.
If the spread is too great, the foundation may not be able to support the weight of the wall and may collapse. This can be a serious safety hazard, so it is important to make sure that the spread is within the correct range.
The other options are incorrect because they do not represent the correct range of spread. Option A, $\frac{2}{3}$ horizontal to 1 vertical, is too great. Option B, 1 horizontal to 1 vertical, is too small. Option C, 2 horizontals to 1 vertical, is also too great.
Therefore, the correct answer is $\boxed{\frac{1}{2}}$ horizontal to 1 vertical.