Which of the following vaccines is known for its use in preventing chickenpox?

The vaccine specifically used to prevent chickenpox is Varicella. This vaccine works by providing immunity against the varicella-zoster virus, which is the causative agent of chickenpox. Vaccination with Varicella helps to reduce the incidence of the disease, as well as the severity of symptoms in those who may still contract chickenpox after vaccination. This vaccine is typically administered in childhood, which plays a crucial role in controlling the spread of the virus within communities.

Other vaccines listed do not target chickenpox. The flu vaccine aims to protect against influenza viruses; Zostavax is designed to prevent shingles, which is a reactivation of the varicella-zoster virus and targeted at older individuals; and the MMR vaccine provides immunity against measles, mumps, and rubella, but not against chickenpox. Thus, Varicella stands out as the correct answer for the prevention of chickenpox.