'viable option' is correct and usable in written English. You can use it when you are looking at different possible solutions or choices and trying to determine which one is the best. For example: "It's important to consider all of the viable options before making a decision."
"It's a viable option.
Suicide is a viable option".
"That's not a viable option".
"Suicide might seem like a viable option.
Incest, for her, is a viable option.
"A spinoff is the only viable option".
Containment was no longer a viable option.
We are a viable option for Darryl".
Self-assessment is a more viable option.
Entrepreneurship is an increasingly viable option.
Hence the only viable option is LOYALTY.
I love the desktop app, it’s always running on my Mac. Ludwig is the best English buddy, it answers my 100 queries per day and stays cool.
Cristina Valenza
Retail Lead Linguist @ Apple Inc.