keeping notes on the condition of equipment used in the procedure conducting an investigation with a group of people conducting the procedure thoroughly one time repeating an investigation several times
I gave my students this multiple choice question: Which procedure best helps to determine whether an investigation is scientifically valid?

Only one answer is correct among these 4 choices:
- keeping notes on the condition of equipment used in the procedure
- conducting an investigation with a group of people
- conducting the procedure thoroughly one time
- repeating an investigation several times

Could you tell me which one is correct?

repeating an investigation several times