Experiments and investigations must be
a. approved
c. repeatable
b. not reproducible
d. accepted

The most important characteristics of a scientific experiment is that it must be reproducible -in other words repeatable (answer c). This means that it must be described in a way as to make it clear to other scientists how the experiment was conducted so that they could repeat it exactly if they decide to.

Experiments and investigations must be repeatable. This is an important characteristic of experiments and investigations to ensure the integrity and accuracy of a study. Other people must be able to conduct the experiment/investigation and obtain similar results to check whether or not a study really has been performed by the researchers and to verify the soundness of the results.