The centripetal force on the stone is provided by the tension in the string.
Its magnitude is F = mv^2/r
Where m is the mass = 1.5 kg
v is the linear velocity of the stone = 4.0 m/s
and r is the radius of curvature = 0.8 m
So F = (1.5)(4.0^2)/0.8 = 19.2 N