The characteristics you provided best describe A: clay. Here's why:
- Feels smooth: Clay has a fine texture and feels smooth to the touch.
- Has moderate amount of air space: While clay particles are very small and compact, they do retain some air pockets.
- Holds water well: Clay has a high water retention capacity due to its fine particles and structure.
- Becomes slippery when wet: Wet clay can become very slippery, which is a characteristic known as "slick."
Therefore, based on these characteristics, the correct answer is A: clay.