Explanation:

- Lighten: To make something less heavy or less severe.

- Cumbersome (Option 1): Means clumsy or hard to manage; it doesn't relate to lightening directly.

- Aggravate (Option 2): Means to worsen or make more severe; not the direct opposite of lighten.

- Trivial (Option 3): Means something of little importance; not related to the opposite of lighten.

- Oppressive (Option 4): Means something heavy or burdensome, often in terms of weight or emotion.

Correct Answer: Oppressive (Option 4)

Because lighten aims to make things lighter, its opposite would be something that is heavy or burdensome, i.e., oppressive.