What class of solids is made up of regularly repeating subunits of tightly packed, orderly arrangements of atoms, ions, or molecules?

The answer is crystalline solid. Crystalline solids have a regular arrangement of particles, because of that, they possess definite geometry.
This means that if you take a look at the crystal under the microscope you would notice that there is a pattern that constantly repeats. This pattern is called a unit cell, and crystal is made up of stacked unit cells.

The crystalline solid is made up of regularly repeating subunits. Crystalline solids have a daily arrangement of particles, as a result of that, they possess definite geometry.
This means that if you're taking a glance at the crystal below the microscope you would notice that there's a pattern that perpetually repeats. This pattern is named a building block, and crystal is formed of stacked unit cells.

Explanation:
The constituents of a crystalline solid structure are organized spatially in a very regular and repetitive manner giving a very ordered structure. This extremely ordered arrangement is usually accompanied by a lowering of potential energy and hence energy in a type of heat is needed to convert it into liquid. This ordered arrangement commonly shows a smaller volume and lower squeezability issue than the liquids.

In crystalline solids the constituents could also be atoms ions or molecules, however, the order arrangement of those constituents that extends over an extended range in the crystal is best referred to as a long-range order.

Crystalline solids are thought-about as anisotropic because the magnitude of property depends upon the direction on that it's measured and crystal solids show precisely that unlike amorphous wherever the properties is freelance of direction.

Ithink it's  c. i'm sure