In the figure shown, BC is a __________.

The correct answer is: ray

We know that a ray is a part of a straight line that has one fixed point but the other point does not have an end i.e., a ray has one terminating end but the other end is extending infinitely. For example, consider the following figure:

In the above figure,  is a ray, where  is the fixed end point and the other point  is extending infinitely i.e., one terminating point is symbolized as  and the infinitely extending part is depicted by an arrow.
Now, consider the given figure:

So, from the figure, it is clear that  is a ray, where  is the fixed end point.

Note that a ray has one end point fixed and the other part is extending infinitely.