Option D sounds great! I'd love to see that in a future release. If that doesn't work out I'd say then go for Option A.
Just as a random thought, maybe you can go with Option A and D both (I'm unaware of how much work it would be, but your call to actually use this idea). You can still have individual tanks build at 1/3 of the time but have the squad based tanks as like some sort of elite unit. Because they are squad based they have that extra cooperation with each other and take down enemies even faster than say building the same number of individual tanks and take longer to receive their promotion since they are acting as a whole. Again, just a random thought.