His partners felt that it was a viable business proposition.

Answer: Option A

Viable : capable of working successfully; feasible.
Impracticable : (of a course of action) impossible in practice to do or carry out.
Inviolable : never to be broken, infringed, or dishonoured.
Unavailable : not able to be used or obtained; not at someone's disposal.
Enviable : arousing or likely to arouse envy.

Antonym of Viable is Impracticable