- Providing support is a kind of function
- A cell wall provides support / protection / shape / structure for a plant cell
- Therefore, the main function of a cell wall is to provide support for a plant cell

Hence, the main function of a cell wall is to provide support for a plant cell
Which of these is the main function of a cell wall?
- Providing support is a kind of function 
- A cell wall provides support / protection / shape / structure for a plant cell