The sum of all interior angles of a regular polygon is equal to sum of all its exterior angles. The number of sides in the polygon is:

1. 4

2. 5

3. 6

4. 8

It is given that the sum of all interior angles of a regular polygon is equal to the sum of all its exterior angles.

According to the properties of a polygon, if there are n sides, then the sum of its interior angles is (2n - 4) x 90° and the sum of its exterior angles is 360°.

⇒ (2n - 4) x 90° = 360°

⇒ (2n - 4) = $\dfrac{360°}{90°}$

⇒ (2n - 4) = 4

⇒ 2n = 4 + 4

⇒ 2n = 8

⇒ n = $\dfrac{8}{2}$

⇒ n = 4

Hence, option 1 is the correct option.