Concept:Temperature gradient is the rate at which temperature changes per unit length along the rod.Explanation:Temperature difference: Δθ=80∘C−20∘C=60∘C.A change of 1∘C is equal to a change of 1K, so Δθ=60K.Thickness of the rod: Δx=0.02m.Temperature gradient is given by ΔxΔθ.Substitute values: 0.02m60K=3000Km−1.So the temperature gradient is 3.0×103Km−1, which corresponds to option B.Answer:3.0×103Km−1 (Option B).