What does it mean for a fetus to be described as "viable"?

Describing a fetus as "viable" means that it has reached a stage of development where it can survive outside of the mother's womb. This typically occurs around the 24th week of gestation, although advances in neonatal care can sometimes allow for survival at earlier stages. When a fetus is viable, it signifies that it has developed sufficient physiological functions, such as lung development and the ability to maintain body temperature, enabling it to live independently with appropriate medical support if necessary.

In contrast, being fully developed mentally and physically refers to a different stage of growth, not solely tied to viability. The idea of being ready for adoption relates primarily to legal and social frameworks rather than biological viability. Lastly, needing medical assistance can occur at any point in a pregnancy, including before the viability stage, and does not accurately reflect the condition of viability itself. Thus, the term "viable" is specifically linked to the ability of a fetus to survive outside the womb, making it the correct choice.