The correct option is A impracticable
Viable means capable of working successfully; feasible or practicable. The opposite of viable is impracticable. Hence, Option A is correct. Among the rest of the options, inviolable means never to be broken, infringed, or dishonoured, unavailable means not able to be used or obtained; not at someone's disposal, enviable means arousing or likely to arouse envy. These options do not express this antonym, hence incorrect.