The correct option is (A).
The rate H at which heat is transferred through the slab is,
(1)directly proportional to the area (A) available.
(2) inversely proportional to the thickness of the slab Δx.
(3) directly proportional to the temperature difference ΔT.
So, H = kA ΔT/ Δx
Where k is the proportionality constant and is called thermal conductivity of the material.
From above we know that, the rate H at which heat is transferred through the slab is directly proportional to the area (A) available.
Area A of solid sphere is defined as,