When the resistance of the sliding rheostat connected to the circuit is minimum, the circuit is a simple circuit of R0.

According to figure b, I max =0.8A,

So R0=UI max = 4v0.8a = 5ω,

The maximum power of the circuit is pmax = uimax = 4v×0.8a = 3.2w；;

When the resistance of the sliding rheostat is maximum, the current in the circuit is minimum.

According to figure b, min = 0.1a,

The total resistance of the circuit is rtotal = uiminimum = 4v0.1a = 40ω.

The maximum resistance of the sliding rheostat is R=R total-r0 = 40ω-5ω = 35ω,

The minimum power of the circuit is p minimum =UI minimum = 4v× 0.1a = 0.4w. 。

So choose B.