A chord of a circle is equal to its radius. The angle subtended by this chord at the minor arc of the circle is :

1. 60°

2. 90°

3. 120°

4. 150°

If chord length AB = radius(OA, OB), then the triangle OAB formed by two radii and the chord is equilateral.

Central angle subtended by the chord = ∠AOB = 60°

∠ACB = $\dfrac{1}{2}$ ∠AOB = $\dfrac{1}{2}$ (60°) = 30°.

Opposite angles of a cyclic quadrilateral ADBC are supplementary.

∠ACB + ∠ADB = 180°

∠ADB = 180° - 30°

∠ADB = 150°.

Thus, angle subtended by this chord at the minor arc of the circle is 150°.

Hence, option 4 is the correct option.