It's 60°
So they have provided some useful information.
Accordingly the length AB = 1 = radius.
So imagine a line from point B to center producing a triangle.
Then since the length AB is already given as 1, and the length from centre to A, and the (imaginary) line B to center is both radius they too equal to 1.
Therefore in creates an equilateral triangle which means all 3 sides are equal, and inside an equilateral triangle one angle = 60°