Centripetal acceleration = (speed²) / (radius of the curve).
-- Radius of the circle in this question = 0.5 meter
-- Circumference = (diameter) x (π) = π meters
-- Speed of the ball on the circle = (2 circumferences / sec) = 2π m/s
Centripetal acceleration = (2π)² / 0.5 = 78.96 m/s²
(about 8.05 g)
The acceleration is directed toward the center of the circle.
To answer the questions that are asked, we don't need to know
the mass of the ball that's revolving. We'd only need to know it
if we were asked to find the centripetal force ... the force on the
ball that keeps it on the circular path, i.e. the tension in the string.