Final answer:
Cassandra, MongoDB, and Couchbase are all NoSQL databases known for their scalability and the ability to store unstructured data, making 'd. All' the correct answer.
Step-by-step explanation:
The correct answer to the question Which among the following is a NoSQL DB? is d. All. Cassandra, MongoDB, and Couchbase are all examples of NoSQL databases. NoSQL databases are designed to provide high scalability and flexibility with the types of data that can be stored. They do not require a fixed schema, and they allow for the storage of unstructured and semi-structured data. Cassandra provides a highly scalable service with no single point of failure, MongoDB is known for its powerful document storage capabilities, and Couchbase provides a distributed design that ensures both high performance and scalability.