Toxicology

Step 1: Analyze the acronym NOAEL based on the provided definition. Step 1: The definition describes the "highest dose" with "no detectable adverse health effects". We need to find the full form that matches this.
Step 2: Break down the acronym and match it to the definition.
Step 2:

No - Matches "no".
Observed - Matches "found by experiment or observation".
Adverse - Matches "adverse".
Effect - Matches "effects".
Level - Matches "highest dose or concentration".
Step 3: Compare with the options.
Step 3: The expansion "No-observed-adverse-effect level" directly corresponds to the acronym and the definition provided. The other options use incorrect words like "essential," "nil," or "adequate."