Based on my knowledge of oblique cabinet projection, the most commonly used pair of scales is C) 3/4 and 2/3.
Oblique cabinet projection is a type of orthographic projection used for representing cabinets and furniture. It utilizes two scaled orthographic views - a front view and a side view.
The front view is drawn to a scale of 3/4, meaning it is drawn at 3/4 of the actual size of the cabinet. This allows more detail to be shown for the front view.
The side view is drawn to a scale of 2/3 of the actual size. Using a smaller scale for the side view allows the entire side view to fit on the drawing.
Since orthographic cabinet projections conventionally use a front view scale of 3/4 and side view scale of 2/3, choice C is the correct answer.
The other scale pairs listed are not as commonly used - 3/4 and 1/2 would make the side view too large, while 1/3 and 1/2 would make the front view too small and the side too large.