Why is calculating only the average insufficient when three measurements differ greatly?

Correct answer: Variation may indicate error or instability

The direct answer is A: variation may indicate error or instability. An average combines measurements into one summary value, but it hides how far the individual readings are from one another. If three readings differ greatly, the scientist should check instrument calibration, reading technique, changing conditions, recording mistakes, or whether the phenomenon itself is variable. Option A is correct because large spread can signal error, poor control, or an unstable process, so the cause must be investigated before trusting the average. Option B is wrong because averages can be useful when measurements are reasonably consistent and are interpreted with spread. Option C is wrong because differing values do not prove that every measurement is wrong; some variation may be genuine. Option D is wrong because the largest value has no automatic claim to be correct. A careful report gives the individual readings and, where suitable, an average together with a measure or description of spread. Memory cue: never let an average hide unusual variation—inspect the data first.