In the given figure, O is the centre of the circle.
If the length of chord AB is equal to the radius of the circle; then ∠ACB is equal to :

1. 30°

2. 45°

3. 60°

4. 40°

If the length of chord AB is equal to the radius of the circle; then △AOB is equilateral triangle.

∠AOB = 60°

We know that,

The angle which an arc subtends at the center is double that which it subtends at any point on the remaining part of the circumference.

∴ ∠ACB = $\dfrac{1}{2}$ ∠AOB

∠ACB = $\dfrac{60^{\circ}}{2}$ = 30°.

Hence, option 1 is the correct option.