What is a placement group?

A. A collection of Auto Scaling groups in the same Region
B. Feature that enables EC2 instances to interact with each other via nigh bandwidth, low latency connections

C. A collection of Elastic Load Balancers in the same Region or Availability Zone

D. A collection of authorized Cloud Front edge locations for a distribution

Explanation:
A placement group is a logical grouping of instances within a single Availability Zone. Using place-ment groups enables applications to participate in a low-latency, 10 Gigabits per second (Gbps) net-work. Placement groups are recommended for applications that benefit from low network latency, high network throughput, or both. To provide the lowest latency, and the highest packet-per-second network performance for your placement group, choose an instance type that supports enhanced networking
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/placement-groups.html