Final answer:
The Varicella-zoster virus (VZV) is responsible for causing shingles, which is a reactivation of the virus that causes chickenpox.
Step-by-step explanation:
The virus responsible for causing shingles is the Varicella-zoster virus (VZV), option (a). VZV is the same virus that causes chickenpox. After an initial chickenpox infection, the virus remains dormant in nerve cells and can reactivate later in life, leading to the development of shingles.