Technician A says an insulator is a substance that resists the flow of electricity. Technician B says the atoms in an insulator are very stable and resist electron movement. Who is right?

They're both correct. The two technicians are right.

Explanation:

Insulators are truly substances that resist the flow of electricity through them.

But the movement of electrons is what causes the flow of current (electricity).

Insulators are incapable of conducting current because they have very stable atoms without electricity-driving valence electrons.

Hence, the two technicians are technically correct.