Step 1: Understanding the Concept:
This question asks for the primary and most common use of a cushion, which is a soft bag of some ornamental material, stuffed with wool, hair, feathers, or other such material, to be used for sitting or kneeling upon, or to rest the head against.
Step 2: Detailed Explanation:
- While a cushion can be used in various informal ways, including for sleeping on a bed or carpet, its designated and primary function in home furnishing is to provide comfort, support, or decoration for seating furniture.
- The most fitting description of its main purpose among the options is to make sitting on a sofa more comfortable, either by supporting the back or being sat upon.
Step 3: Final Answer:
The primary use of a cushion is for sitting comfortably on a sofa.