Why do many beaches have loose sand instead of solid rock or mud?

Answer: Rivers and waves break rock and shell into small, rounded grains over a long time.

Rivers and waves break rock and shell into small, rounded grains over a long time. ✓ — Correct! Sand is mostly weathered rock and shell material. Rivers carry small pieces to the sea, and waves tumble them against one another for a long time. This grinding and smoothing makes the small, rounded grains we call sand; quartz is common because it resists weathering.

Salt in seawater forms crystals that waves wash up onto the beach. — Wrong. Seawater does contain dissolved salts, and salt can form crystals when water evaporates, so the starting point is real. But those crystals dissolve again in water and do not build up as durable beach sand. Most sand is broken-down rock and shell that waves and rivers have worn into grains.

Mud carried by rivers dries on the shore and cracks into loose sand grains. — Wrong. Mud does reach the sea and can make muddy shores, so this idea gets part of the picture right. But drying mud does not turn it into durable sand grains. Beach sand forms from rock and shell fragments broken down by water and waves.