a and b are real numbers, namely the radius of two balls. They forgot to define B_a, but typically B_a(x) := {y in X : d(x, y) < a} where (X,d) is the metric sp

a and b are real numbers, namely the radius of two balls. They forgot to define B_a, but typically B_a(x) := {y in X : d(x, y) < a} where (X,d) is the metric space. I agree with you though, I don't think it's option (a)