Step 1: Viability means a fetus is developed enough to survive on its own outside the uterus, independent of the mother. The question asks for the legally recognised age at which this becomes possible.
Step 2: In medico-legal practice this threshold is set at 210 days, roughly the completion of the 7th lunar month of intrauterine life. Survival is occasionally reported from 180 days, but such fetuses are usually too immature to live.
Step 3: Of the choices, only 210 days matches this standard. The larger figures of 220, 230 and 240 days exceed the accepted point of independent existence and are therefore incorrect.
Step 4: Confirming the age, the talus shows its primary ossification centre at the end of the 7th month, a recognised sign that viability has been reached.
\[\boxed{\text{210 days}}\]