Answer:
BC
Explanation:
The definition of a diameter is a line that connects 2 points on the circumference (i.e a chord) which also passes through the center of the circle. (see attached)
In this case, we assume that BC is a straight line, we can see that
a) It is a line that connects 2 points on the circumference
b) also passes through the center of the circle at Point A
Hence BC is the diameter.