At a vertex of a regular polygon, exterior angle is 120°. Then the number of sides of this polygon is:

1. 3

2. 4

3. 5

4. 6

Given, exterior angle = 120°.

The sum of the exterior angles of any convex polygon is 360°. For a regular polygon, all exterior angles are equal.

Let the number of sides of polygon be n.

By formula,

For a regular polygon,

Number of sides in it = $\dfrac{360°}{\text{Each exterior angle}} = \dfrac{360°}{120°}$ = 3.

Hence, option 1 is the correct option.