Answer: Option B
$$\eqalign{
& {\text{External}}\,{\text{radius}} = 4\,cm \cr
& {\text{Internal}}\,{\text{radius}} = 3\,cm \cr
& {\text{Volume}}\,{\text{of}}\,{\text{iron}} \cr
& = \left( {\frac{{22}}{7} \times \left[ {{{\left( 4 \right)}^2} - {{\left( 3 \right)}^2}} \right] \times 21} \right)c{m^3} \cr
& = \left( {\frac{{22}}{7} \times 7 \times 1 \times 21} \right)c{m^3} \cr
& = 462\,c{m^3} \cr
& \therefore {\text{Weight}}\,{\text{of}}\,{\text{iron}} = \left( {462 \times 8} \right)gm \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 3696\,gm \cr
& \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 3.696\,kg \cr} $$