See this wordreference.com forum on ma vs bensi. I think ma is best. Perché would mean because they aren't electric. I can imagine the silly idea that electric eels power the hovercraft where you would use perché but omit the "non," but with the "non" I don't see what on earth it's supposed to mean with perché.
Also note that anguille is feminine plural, so your adjective should match that: elettriche.
In this sentence the "elettrici" is the clue to the form of essere. Because elettrici is the plural form of elettrico we know there are more than one. Luckily (for the world mostly) there is only one of me (or io), so we know sono is being used in this sentence to refer to "They". Whereas, I believe "Sono elettrico?" would ask "Am I electric?". I hope this helps.
An' you ain't seen nothin' yet. At the beginning Duo has some of the most outlandish sentences. As your vocabulary increases things get better but there is also some method behind the madness. It keeps you on your toes and by avoiding cliches and commonplace sentences you have to pay attention.
Interested in more try here: ❤❤❤❤ Duolingo Says (@shitduosays) | Twitter https://twitter.com/shitduosays
On a more academic note here is a useful post: >https://www.duolingo.com/comment/4821654
Ciao. Since we're only given the verbo, sono, and the adjective in its plural form, I wrote "are those electric", thinking of appliances rather than people (they). Earlier, for fruit, I wrote "they" and it was marked wrong, and the explanation was that I should have used ESSI-ESSE. Therefore, why is it correct to use THEY sono elettrici? Thanks!