To find the straight-line distance from A's office to D's office, we need to find the distance between points A and D on a coordinate plane.

Let's assume that A is located at the origin (0, 0) of the coordinate plane. Then B's office is located at point (4, 0), C's office is located at point (4, 6), and D's office is located at point (8, 6).

Using the distance formula, we can find the distance between A and D:

distance = sqrt[(x2 - x1)^2 + (y2 - y1)^2]

where (x1, y1) are the coordinates of A and (x2, y2) are the coordinates of D.

Substituting the values, we get:

distance = sqrt[(8 - 0)^2 + (6 - 0)^2] = sqrt[64 + 36] = sqrt[100] = 10

Therefore, the straight-line distance from A's office to D's office is 10 km.