Under which conditions will the consumer see the message?

A producer just sent a message to the leader broker for a topic partition. The producer used acks=1 and therefore the data has not yet been replicated to followers.

Under which conditions will the consumer see the message?
A . Right away
B . When the message has been fully replicated to all replicas
C . Never, the produce request will fail
D . When the high watermark has advanced

Answer: D

Explanation:

The high watermark is an advanced Kafka concept, and is advanced once all the ISR replicates the latest offsets. A consumer can only read up to the value of the High Watermark (which can be less than the highest offset, in the case of acks=1)