AWS Certified Solutions Architect - Professional (#503)

Doug has created a VPC with CIDR 10.201.0.0/16 in his AWS account. In this VPC he has created a public subnet with CIDR block 10.201.31.0/24. While launching a new EC2 from the console, he is not able to assign the private IP address 10.201.31.6 to this instance. Which is the most likely reason for this issue?

Private address IP 10.201.31.6 is currently assigned to another interface
Private IP address 10.201.31.6 is reserved by Amazon for IP networking purposes.
Private IP address 10.201.31.6 is blocked via ACLs in Amazon infrastructure as a part of platform security.
Private IP address 10.201.31.6 is not part of the associated subnet's IP address range.