Ray ab and ray bc are on the same line (fill in the blank) a. always b. sometimes c. never

C. Never

This is because they should be parallel if the are two separate rays.

I would say that they are sometimes on the same line. Think of the following scenario: a line, AC, is bisected by point B.  You do indeed have two rays, each representing a part of the line. 

Another scenario, think of a standard number line. Say AB goes from -1 to -3 and then continues to the left. BC goes from 2 to 4 and then continues to the right. These are two rays, each pointed in opposite directions, but on the same line.