The hdfs dfsadmin command in HDFS is used to add or remove data nodes dynamically. It provides administrative functions for managing the Hadoop Distributed File System, including the addition or decommissioning of data nodes.

YARN (Yet Another Resource Negotiator) in Hadoop clusters helps in identifying bottlenecks and optimizing resource allocation. It manages and allocates resources efficiently, allowing various applications to run simultaneously on the cluster.

In advanced Hadoop performance tuning, adjusting the Garbage Collection threshold for task JVM reuse is crucial. Garbage Collection helps manage memory and reclaim unused resources, impacting the overall performance of Hadoop tasks. Tweaking this threshold can optimize resource utilization.

The Hadoop Diagnostics Tool (HDT) is used for diagnosing network topology and speed between nodes in HDFS. It helps administrators identify potential issues related to network performance and data transfer within the Hadoop cluster.

Oozie workflows are based on a declarative programming model. In a declarative approach, users specify what needs to be done and define the desired state, and Oozie takes care of coordinating the execution of tasks to achieve that state.

Java is primarily used for writing MapReduce jobs in Hadoop's native implementation. Hadoop's MapReduce framework is implemented in Java, making it the language of choice for developing MapReduce applications in the Hadoop ecosystem.

The heartbeat signal between DataNode and NameNode serves as a health check for DataNodes. It allows the NameNode to verify the availability and health status of each DataNode in the cluster. If a DataNode fails to send a heartbeat within a specified time, it is considered dead or unreachable, and the NameNode initiates the block replication process to maintain data availability.

In MapReduce, the Shuffle phase involves sorting and merging the intermediate data from mappers before sending it to the Reducer. This phase is critical for optimizing data transfer and reducing network overhead.

Examining the first few lines of a task's log file is significant in debugging a Hadoop application as it helps diagnose task failures. The log provides valuable information about the execution context, errors, and exceptions encountered during the task, aiding developers in identifying and resolving issues.

The correct option is 'Coordination.' Oozie workflows can be optimized using coordination for effective resource utilization and scheduling. Coordination allows for better synchronization and control of job execution, ensuring that dependencies are met before starting subsequent jobs.