Answer: time
Step-by-step explanation:
A scientific research involves a systematic approach. It is based upon implementation of a scientific methodology. The results obtain after the process are liable to acceptability or rejection depending upon the reliability and validity.
A thorough analytical research process requires time, money and technology. These three factors support a valid and exceptable research outcome.
According to the given situation, here it is already mentioned that tortoise live hundreds of years. But the experimenter wants to know whether the tortoise live for even longer time based upon the supplement. The experimenter just have got time of five years which is limited for exploring the outcome of the research. Hence, time is the limitation in the experiment.