What is the fixed size allocated to a VPC for IPv6 CIDR blocks?

Answer options:

A./64
B. /56
C. /40
D./32

Answer - B
This is mentioned in the AWS documentation
For IPv6, the VPC is a fixed size of /56 (in CIDR notation). A VPC can have both IPv4 and IPv6 CIDR blocks associated to it.
For more information on VPC’s, please refer to the below link:
https://aws.amazon.com/vpc/faqs/