"Los amigos son fundamentales."
Translation:The friends are essential.
"The friends are fundamental." That's not a normal sentence. I honestly thought it was a trick question having that awkward translation in as an response.
Fundementales can also mean "essential" and in this context it makes a ton more sense
The multichoice example had "The friends are fundamental" as the only logical answer. But of course, the above translation is what is meant. Evidently, brevity is not customary in Spanish- or in some countries.