When should data be recorded during an experiment?

Correct answer: At the time of observation or measurement

The direct answer is C: at the time of observation or measurement. Data should be written immediately, or as close as safely possible to the moment when the observation is made. This preserves the actual reading and reduces memory mistakes, accidental changes, and confusion between trials. Step by step: perform one observation or measurement; read the instrument carefully; write the value at once; include the unit and, when useful, the trial number and conditions; repeat for the next reading. Option A is wrong because waiting until the next day may cause the researcher to forget or mix up values. Option B is wrong because data are evidence used to form a conclusion, so they must exist before the conclusion. Option C is correct because recording during observation gives the most reliable primary record. Option D is wrong because relying on memory is not a dependable scientific method. A value without a unit may also be unclear, so units should be recorded. Memory cue: observe, measure, record—then analyse. Never create or rewrite data merely to make a pattern look better.