I voted for A because I thought it had the strongest design, the best proportions, showing more or less the Golden Ratio (look it up).

The boxy 1:1 ratios shown in the others result, to my eye, in weak designs. Though in many tartans a weak design can be saved by great use of colour.

(Reducing a tartan to B&W will tell you how strong the design itself is.)

But C is also quite pleasant.

A great example of the use of proportions is a comparison of many traditional tartans to those designed by the so-called Sobieski brothers. Their efforts were nearly always boxy and inelegant.