Final answer:
The ESL teacher is primarily using participant observation to make notes on the English Language Learners' progress as it is observed during the natural course of the school day.
Step-by-step explanation:
The ESL teacher is primarily using participant observation to make notes on the English Language Learners' (ELLs) progress as it is observed during the natural course of the school day. Participant observation involves immersing oneself in a cultural context and making observations and notes about what occurs. It allows the teacher to gather information about how the ELLs use language naturally in different settings.