An "alternative" is a choice or a possible solution to a problem.
A "viable alternative" is a choice or possible solution which might work.
To illustrate this with an example, imagine that you need to get to work but you don't want to drive a car. Walking to work is not a "viable alternative" because you live too far away from your office. But maybe riding a bicycle is a "viable alternative".
Alternatives can be "viable" or not "viable" in a few different ways:
- economically viable
- politically viable
- commercially viable (able to be sold)