You currently have 9 EC2 instances running in a Placement Group. All these 9 instances were initially launched at the same time and seem to be performing as expected. You decide that you need to add 2 new instances of different instance types to the group. However, when you attempt to do this, you receive a `capacity error`. Which of the following actions will most likely fix this problem? Choose the correct answer from the options below.

Answer options:

A.Make a new Placement Group and launch the new instances in the new group. Make sure that the Placement Groups are in the same subnet.
B.Replace instances of the same type. Stop and restart the instances in the Placement Group and then try the launch again.
C.Request a capacity increase from AWS as you are initially limited to 10 instances per Placement Group.
D.Make sure that all the instances are the same size and then try the launch again.

Answer – B
Refer link
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/placement-groups.html#concepts-placement-groups
"If you try to add more instances to the placement group later, or if you try to launch more than one instance type in the placement group, you increase your chances of getting an insufficient capacity error."
This is also given in the AWS DocumentationInsufficient Instance Capacity.
Description
You get the InsufficientInstanceCapacity error when you try to launch a new instance or restart a stopped instance.
Cause
If you get an InsufficientInstanceCapacity error when you try to launch an instance or restart a stopped instance, AWS does not currently have enough available On-Demand capacity to service your request.
Solution
To resolve the issue, try the following:
Wait a few minutes and then submit your request again; capacity can shift frequently.
Submit a new request with a reduced number of instances. For example, if you`re making a single request to launch 15 instances, try making 3 requests for 5 instances or 15 requests for 1 instance instead.
If you`re launching an instance, submit a new request without specifying an Availability Zone.
If you`re launching an instance, submit a new request using a different instance type (which you can resize at a later stage). For more information, see Changing the Instance Type.
If you are launching instances into a cluster placement group, you can get an insufficient capacity error. For more information, see Placement Group Rules and Limitations.
Try purchasing Reserved Instances, which are long-term capacity reservation. For more information, see Amazon EC2 Reserved Instances.
Options A, C and D are all invalid because none of these options will help resolve the error.
For more information on this error, just browse to the below URL:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-capacity.html