Explanation:

The word 'onerous' means involving a great deal of effort, trouble, or difficulty; burdensome. Therefore, its opposite would be something that is not difficult or burdensome. Option C ('easy') perfectly fits this description. Options A ('austere') and B ('spartan') relate to severity or simplicity, not ease. Option D ('trying') is a synonym for difficult, making it an antonym of 'easy' and a near-synonym of 'onerous'.