You are developing a system that will be sending messages to an SQS queue. Another application will be running on an EC2 Instance that will be used to process the messages. Which of the following are BEST practices for making COST effective use of the SQS queues? Choose 2 answers from the options given below.

Answer options:

A.Use short polling for SQS queues.
B.Use long polling for SQS queues.
C.Group the SQS API operations in batches.
D.Use single queue operations.

Answer – B and C
This is given in the AWS Documentation.
Reducing Amazon SQS Costs
The following best practices can help you reduce costs and take advantage of additional potential cost reduction and near-instantaneous response.
Batching Message Actions
To reduce costs, batch your message actions:
To send, receive, and delete messages, and to change the message visibility timeout for multiple messages with a single action, use the Amazon SQS batch API actions.
To combine client-side buffering with request batching, use long polling together with the buffered asynchronous client included with the AWS SDK for Java.
Note
The Amazon SQS Buffered Asynchronous Client doesn`t currently support FIFO queues. 
Because of what is mentioned in the AWS Documentation as best practices, other options are invalid.
For more information on reducing costs for SQS, please refer to the below URL-
https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/reducing-costs.html