Answer: The right answer is<em> magnum opus</em>, in singular, and <em>magna opera</em>, in plural.
Explanation: Just to elaborate a little on the answer, it can be added that <em>magnum opus</em> literally means "great work," and it is an expression used to describe a literary piece, a musical composition, a painting, a film, or other example of artistic creativity and genius that stands as the best and most important achievement of its creator.
