Answer :

Solution :

t_s ∝ V/A

cube of edge length = a

Volume and area of cube: V = a³, A = 6a²

cylindrical riser with height = h , radius = r

Volume and area of riser: V = πr²h , A = 2πrh

(t_s)_Riser ≥ (t_s)_casting

${\left(\frac{V}{A}\right)}_{riser}\ge {\left(\frac{V}{A}\right)}_{cube}$

${\left(\frac{\pi r^{2}h}{2\pi rh}\right)}_{riser}\ge {\left(\frac{a^3}{6a^2}\right)}_{cube}$

r ≥ a/3

r = a/3

from optionsr = a/2 also satisfies.