What are private VLANs?

What are private VLANs?

A.
tools that allow segregating traffic at Layer 3, turning broadcast segments into non-broadcast,
multi-access-like segments

B.
tools that allow segregating traffic at Layer 2, turning broadcast segments into non-broadcast,
multi-access-like segments

C.
tools that allow segregating traffic at Layer 2, turning non-broadcast, multi-access-like
segments into broadcast segments

D.
tools that allow segregating traffic at Layer 3, turning non-broadcast, multi-access-like
segments into broadcast segments