Answer:
Scientists reached a decision about when to begin the experiment.
Explanation:
A decision was reached by scientists (passive voice, i.e. the subject A decision receives the action.)
Scientists reached a decision (active voice, i.e. the subject of sentence Scientists performs the action)
...when the experiment would begin sounds as if it would begin by itself, without any involvement on the part of scientists.
...when to begin the experiment shows the active role of the subject Scientists.
