The kafka-consumer-groups.sh command is the standard administrative tool for managing and inspecting consumer groups. When used with the --describe flag, it provides detailed information about a consumer group, including the current offset for each partition, the log-end-offset (the offset of the last message), and the calculated lag. Lag is the difference between the log-end-offset and the consumer's current offset, indicating how far behind the consumer is. This makes it the correct tool for quickly checking consumer lag as requested in the scenario.

A. bin/kafka-consumer-lag.sh: This is not a standard command-line tool provided with Apache Kafka or the Confluent Platform. It does not exist in the official distribution.

C. bin/kafka-consumer-group-throughput.sh: This command does not exist in the standard Apache Kafka or Confluent Platform toolset. Throughput is a different metric from lag.

D. bin/kafka-reassign-partitions.sh: This tool is used for cluster administration, specifically for moving topic partitions between brokers. It is not used for monitoring consumer performance or lag.

1. Confluent Documentation: "kafka-consumer-groups". The documentation explicitly states that the --describe option can be used to "list the topic partitions, current-offset, log-end-offset, and lag for a consumer group."

Source: Confluent Platform Documentation, CLI Tools, kafka-consumer-groups, "Describe a consumer group" section.

2. Apache Kafka Documentation: "Kafka Operations - Consumer Groups". The official Apache Kafka documentation details the usage of kafka-consumer-groups.sh. The "Describing a Consumer Group" section shows an example output with columns for CURRENT-OFFSET, LOG-END-OFFSET, and LAG.

Source: Apache Kafka 3.6 Documentation, Section 4.4 "Consumer Groups".

3. Gwen Shapira, Todd Palino, Rajini Sivaram (2021): Kafka: The Definitive Guide, 2nd Edition. Chapter 11, "Administering Kafka," describes the kafka-consumer-groups.sh script as the primary tool for inspecting consumer group state, including offsets and lag.

Source: O'Reilly Media, ISBN: 978-1098108035, Chapter 11, "Consumer Groups" section.