Final answer:
Parallel lines in mathematics are lines that never intersect and are always the same distance apart. There are two types: those that are out of phase with each other and those that are in phase with each other.
Step-by-step explanation:
In mathematics, parallel lines are lines that never intersect and are always the same distance apart. They have the same slope and run in the same direction. There are two types of parallel lines: those that are out of phase with each other (c) and those that are in phase with each other (d).