You have a requirement to create a subnet which will have the ability to host 512 usable addresses. Which of the below network masks would you use to ensure that the ability to host this many IP addresses is as accurate as possible

Answer options:

A./21
B./22
C./23
D./24

Answer – B
If you have the subnet mask of /23, you will have exactly 512 hosts; out of this 5 IPs are not available to use, because those are reserved IPs. So, you will end up with 507 hosts which is not meeting our requirement. 
Hence, the correct answer would be option
B. 
The first four IP addresses and the last IP address in each subnet CIDR block are not available for you to use, and cannot be assigned to an instance. For example, in a subnet with CIDR block 10.0.0.0/24, the following five IP addresses are reserved:
10.0.0.0: Network address.
10.0.0.1: Reserved by AWS for the VPC router.
10.0.0.2: Reserved by AWS. The IP address of the DNS server is always the base of the VPC network range plus two; however, we also reserve the base of each subnet range plus two. For VPCs with multiple CIDR blocks, the IP address of the DNS server is located in the primary CIDR. For more information, see Amazon DNS Server.
10.0.0.3: Reserved by AWS for future use.
10.0.0.255: Network broadcast address. We do not support broadcast in a VPC, therefore we reserve this address.
Please check the below link to know more about it:
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html