The operational definition in an experiment was unclear and different observers interpreted it differently. What is the best improvement?

Correct answer: Define clear and measurable criteria

The direct answer is A: define clear and measurable criteria. An operational definition explains exactly how a scientist will recognize or measure a variable. For example, instead of saying that a plant is healthy, the study might define health as a particular height, number of leaves, or mass measured in a stated way. If observers use different meanings, their records cannot be compared reliably. Option A is correct because pre-decided, observable, measurable rules reduce observer differences and improve consistency. Option B is wrong: using fewer observers does not repair an unclear definition and may reduce useful checking. Option C is wrong because an average can summarize numbers but cannot decide what was measured or remove inconsistent meanings. Option D is wrong and scientifically unacceptable because deciding the outcome afterward invites bias. The correct procedure is to define the variable and its measurement rule before collecting results. Memory cue: first define how to measure, then measure.