She decided to finish the work before it became onerous.

Answer: Option B

Onerous : (of a task or responsibility) involving a great deal of effort, trouble, or difficulty.
Unfamiliar : not known or recognized.
Light : requiring little mental effort; not profound or serious.
Burdensome : difficult to carry out or fulfil; taxing.
Unique : being the only one of its kind; unlike anything else.

Antonym of Onerous is Light