Explanation:

- Weighty: This means something of substantial importance or heavy in physical weight.

- Option 1: important: This means something having great significance or value. It's similar to "weighty" in terms of significance.

- Option 2: ponderous: This refers to something heavy or slow, either physically or metaphorically. It aligns with "weighty" in the heaviness aspect.

- Option 3: light: This means something that is not heavy, both physically and figuratively. It is the opposite of "weighty."

- Option 4: low-necked: This refers to clothing with a low-cut neckline and is unrelated to the concept of weight.

The correct antonym for "weighty" is:

- Option 3: light