I've found out that there are several prepositions for the word alternative that all seem to be correct, however, I think there should be a best choice. Do these prepositions affect the meaning in some way?
For:
Ensure that the text alternative for CAPTCHA images identifies the type of task and what is required from the user to complete it
To:
This solution is far from perfect, but it is a potential alternative to Captcha nonetheless.
Of:
In this tutorial, we discussed an alternative of captcha, a plugin called GASP (Growmap Anti spambot Plugin).
Personally, as in my recent article, I do prefer to use "To" as preposition.
Best Answer