A bimetallic strip is made of aluminium and steel (α_Al > α_steel) . On heating, the strip will

First of all understand the bimetallic strip, when two strips of equal length but of different materials, means different coefficient of linear expansion, join together then this is known as bimetallic strip. This strip has the property of bending on heating due to unequal linear expansion of two metals.

Both strips of Al and steel are fixed together initially in bimetallic strip. When both are heated then expansion in steel will be smaller than aluminium.

So, Al strip will be convex side and steel on concave side.

Therefore option (d) is correct.