"An easy work" is definitely not something we would say in American English. Anywhere else?
An easy job. It was accepted, but the phrase "easy work" should not be one of the correct answers (unless there are people from other English-speaking nations that would like to disagree)
Definitely not said in British (and presumably International?) English either. "an easy work" sounds weird, you'd never hear a native speaker say that here. "An easy life" would be more common but that doesn't necessarily carry any implications towards the work they do. You could perhaps say "My nephew's work is easy", but really in the context of this particular sentence I'd agree that "My nephew has an easy job" should be the only answer accepted.
It only makes sense as a work of art. My immediate thought was a piece to learn on a musical instrument.
This sentence, without the article ("a"), makes perfect sense. "My nephew has easy work."