Answer: ApproximateNumberOfMessagesVisible

ApproximateNumberOfMessagesVisible - This is a CloudWatch Amazon SQS queue metric. The number of messages in a queue might not change proportionally to the size of the Auto Scaling group that processes messages from the queue. Hence, this metric does not work for target tracking. Incorrect options: With target tracking scaling policies, you select a scaling metric and set a target value. Amazon EC2 Auto Scaling creates and manages the CloudWatch alarms that trigger the scaling policy and calculates the scaling adjustment based on the metric and the target value. It is important to note that a target tracking scaling policy assumes that it should scale out your Auto Scaling group when the specified metric is above the target value. You cannot use a target tracking scaling policy to scale out your Auto Scaling group when the specified metric is below the target value. ASGAverageCPUUtilization - This is a predefined metric for target tracking scaling policy. This represents the Average CPU utilization of the Auto Scaling group. ASGAverageNetworkOut - This is a predefined metric for target tracking scaling policy. This represents the Average number of bytes sent out on all network interfaces by the Auto Scaling group. ALBRequestCountPerTarget - This is a predefined metric for target tracking scaling policy. This represents the Number of requests completed per target in an Application Load Balancer target group.

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