Answer: D) 4 : 5
Since circumference $= 2\pi r$, the ratio of circumferences equals the ratio of radii. If $C_1 : C_2 = 4 : 5$, then $r_1 : r_2 = 4 : 5$.
Circumference is directly proportional to radius ($C = 2\pi r$), so the ratio of radii is the same as the ratio of circumferences. Examiners expect students to recall this direct relationship instantly. Do not confuse with area (where the ratio would be squared: $16:25$).