The internal voltage is 8V-4V=4V,

Then the current is: r = 41= 4A in I = u.

Then the current of the wire in the magnetic field is: I'=4A-2A=2A.

According to the equilibrium condition: f = mgsin37 = 3n.

F=BIL, so: b = 32×1=1.5t.

So choose: a.