Final answer:
When two lines are perpendicular, they form a 90° angle between each other.
Step-by-step explanation:
When two lines are perpendicular, they form a 90° angle between each other. So, if line OS is perpendicular to another line, it means that the two lines form a 90° angle. This is option b: 'They are perpendicular, forming a 90° angle between each other.'