Answer:
Scientists reached a decision about when to begin the experiment.
Step-by-step explanation:
Changing of a sentence from passive to active voice or vice versa involves the situational changes in the sentence of both the subject and the object (if there are any). From the example given in the question, the sentence in the passive voice is-
A decision was reached by scientists about when the experiment would begin.
To change this sentence into an active voice, the identification of the subject and object is needed. And then, in an active voice, the subject will be at the beginning of the sentence.
'Decision' is the object in the sentence while the 'scientists' signify the subject(s).
Thus, the new sentence in the active voice will become-
Scientists reached a decision about when to begin the experiment.