For this question
Which of these CIDR blocks and/or IP addresses are invalid for a private VPC or subnet on AWS? (Choose 3)
A. 10.2.4.255 –> If I create a VPC with this CIDR block 10.2.0.0/16 I could have the 10.2.4.255 in an instance
B. 126.96.36.199 –> This is clear, I could create a VPC with 188.8.131.52/24
C. 172.31.4.2/15 –> The greatest CIDR block allowed is /16
D. 192.168.1.2 with subnet mask 255.255.255.0 –> I cannot specify this in a VPC or subnet, I can change instead for 192.168.1.0/24 or 192.168.0.0/16
The answer marks the A as an invalid, but If I create a VPC with this CIDR block 10.2.0.0/16 I could have the 10.2.4.255 in an instance, Couldn’t I?
You are right that 10.2.4.255 is a perfectly usable address if you create a VPC using 10.2.0.0/16 then maybe a single subnet for 10.2.0.0/16 to allocate all those addresses to that single subnet. I’ve updated that question to remove that as an invalid answer.