The correct answer is A. One-fourth of its width.
A shallow foundation is a foundation that is less than one-fourth of its width in depth. It is typically used for light-weight structures, such as single-family homes. Shallow foundations are less expensive to construct than deep foundations, but they are not as stable.
A deep foundation is a foundation that is more than one-fourth of its width in depth. It is typically used for heavy-weight structures, such as commercial buildings and high-rise apartments. Deep foundations are more stable than shallow foundations, but they are also more expensive to construct.
The other options are incorrect because they do not meet the definition of a shallow foundation. Option B, half of its width, is too deep to be considered a shallow foundation. Option C, three-fourths of its width, is also too deep to be considered a shallow foundation. Option D, equal to its width, is not a valid option because a foundation cannot be equal to its width in depth.