The correct answer is: B. 42 diameters from the inner edge of the column
The purpose of extending the bottom bars under the columns is to provide additional support for the column and to prevent the column from overturning. The bars are extended into the interior of the footing slab to a distance greater than 42 diameters from the inner edge of the column. This ensures that the bars are anchored in the concrete and that they will not pull out of the footing slab.
Option A is incorrect because it is the distance from the center of the column, not the inner edge. Option C is incorrect because it is the distance from the outer edge of the column, not the inner edge. Option D is incorrect because it is the distance from the center of the column, which is not sufficient to provide adequate support for the column.