The correct option is
C Easy
The word 'onerous' refers to a task or responsibility that involves a great deal of effort, trouble or responsibility.
Option A, egregious refers to something that is outstandingly bad or shocking.
Option B, effusive refers to showing or expressing gratitude, pleasure or approval.
Option C, easy refers to something that can be achieved without great effort.
Option D, efficient refers to achieving maximum productivity with minimum wasted effort.
Thus, according to the meaning, the word 'easy' is the most different in meaning from the word 'onerous'.