Fluency in reading can be defined as:

Fluency in reading is primarily characterized by the ability to read text accurately and quickly, which allows readers to focus on understanding the meaning of the text rather than getting bogged down by decoding individual words. When students are fluent readers, they can recognize words automatically and can read with appropriate speed and expression, which ultimately enhances comprehension. This skill is essential for developing overall literacy and enables learners to engage more deeply with the material they are reading.

The other options presented do not address fluency in the context of reading. Writing essays pertains to composition skills, understanding complex texts relates more to reading comprehension, and memorizing vocabulary focuses on word recognition but does not incorporate the speed and expression components that define reading fluency.