Answer: 2 inch dimension will give smallest increase.
Explanation:
Length = 3 in
width = 2 in
height = 6 in
Extra cardboard means to find surface area
on doubling the length
length = 6 In
width = 2 In
Height = 6In
Surface area for the above dimensions = 2 [ 6x2+2x6+6x6] = 120 sq in
On doubling the width
length = 3 in
width = 4 in
Height = 6 inch
Surface area for the above dimensions= 2 [ 3x4+4x6+6x3] = 2[54] = 108 sq inches
On doubling height
Length =3 in
width = 2 in
Height = 12 in
Surface area for above dimensions = 2 [ 3x2+2x12+12x3] = 2[6+24+36] = 132 sq inch
On doubling width surface area is minimum.