Answer
Option c:
Do both [See if the object attracts a charged plastic rod] and [See if the object repels a charged glass rod.].
Work Step by Step
For a charge to exist it must follow the two following properties:
1.) It must attract all opposite charges.
2.) It must repel like charges.
Thus, in a world where there are only plastic and glass charges, for a glass charge to be considered a charge, it must attract plastic charges AND repel glass charges.
Option c. is correct:
Do both [See if the object attracts a charged plastic rod] and [See if the object repels a charged glass rod.].
