Which one is a secure way to configure that the instance can be accessed only from this IP?

A user is accessing an EC2 instance on the SSH port for IP 10.20.30.40.

Which one is a secure way to configure that the instance can be accessed only from this IP?
A . In the security group, open port 22 for IP 10.20.30.40
B . In the security group, open port 22 for IP 10.20.30.40/32
C . In the security group, open port 22 for IP 10.20.30.40/24
D . In the security group, open port 22 for IP 10.20.30.40/0

Answer: B

Explanation:

In AWS EC2, while configuring a security group, the user needs to specify the IP address in CIDR notation. The CIDR IP range 10.20.30.40/32 says it is for a single IP 10.20.30.40. If the user specifies the IP as 10.20.30.40 only, the security group will not accept and ask it in a CIRD format.

Reference: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-network-security.html