Tesserae of semiprecious stones were set into the plaster at an angle in order to a. fit into a particular space. c. reflect light better. b. create an imperfection. d. keep them from being stolen.

c. reflect light better.

Explanation:

We are talking of mosaics that were made by teams of artisans created on plaster. It was common to use semi-precious stones that could be carefully placed by an artisan so that they would better reflect the light, if slanted a bit by design, some of the tiny mosaic semi-precious stones would catch and reflect light.

because other regions has other things that egypt doesnt have.