What material is NOT acceptable for the volleyball cover?

The material not acceptable for the volleyball cover is canvas. Official volleyballs used in competitions must adhere to specific material standards to ensure consistency in play and performance. Synthetic leather, textured leather, and suede leather are materials that are generally deemed suitable for volleyball covers due to their durability and ability to provide a consistent touch and feel during gameplay.

Canvas does not meet these requirements as it tends to be less durable and can absorb moisture, which negatively impacts the ball's performance and handling. Therefore, the use of canvas would not be appropriate for official volleyballs, making it the correct answer in this context.