Answer: Trigger based on the size of elements in bytes

Dataflow supports three main types of triggers: 1. Time-based triggers, which emit results based on time. 2. Data-driven triggers, which emit results when a window receives a certain number of data elements. 3. Composite triggers, which combine multiple time-based or data-driven triggers in a logical manner. The option mentioning a trigger based on the size of elements in bytes is not one of the supported types. Reference: [Google Cloud Dataflow Triggers](https://cloud.google.com/dataflow/model/triggers)

Author: LeetQuiz Editorial Team