Translation:We will choose the best cat, because it is free.
The purpose of Duolingo isn't to assess the standard of your Irish, or your English, it's to teach basic Irish, using a process of staged repetition, and a basic vocabulary. Allowances have been made for people who already have some Irish, by adding alternative answers, but that should be a two way street - you should make allowances for Duolingo and learn it's vocabulary too. While "as" can be manually added as an alternative in this sentence, it isn't a synonym for "because" in all cases, and Duolingo teaches mar as "because".
When translating from Irish to English, I would suggest not. There really isn't any good reason not to use the most straightforward translation "best", which is how is fearr will usually be interpreted when used in a comparison like this.
You also have the fact that the "most" is superfluous - what is the difference between "the most preferable cat" and "the preferable cat"? You might argue that you'd use "the preferable cat" for selecting from two (the better cat), but "the most preferable cat" for selecting from more than two (the best cat), but in both Irish and English, you can use is fearr/"the best" for selecting from two ( is fearr é ná tada - "it's better than nothing").
You can use is fearr to translate "preferable" from English to Irish, but it's not the best choice (an rogha is fearr) for a direct translation back from Irish to English.