D analysis shows that when the friction reaches the maximum, the acceleration is: a = 3 μ g; C analysis shows that when the friction reaches the maximum, the acceleration: a ′ = 2 μ g,

On the whole, according to Newton's second law, f = (m+m+2m+4m) a ′ =16 μ g, and the friction between BDs is f = f-4ma ′ = 8 μ g, which leads to relative sliding.

Therefore, the maximum resultant force on the ACD object is 3 μ g = (m+m+2m) a ",and the maximum pulling force fmax = (m+m+2m+4m) a" = 6 μ g..

So choose: B.